Subject: Re: [PATCH v7 1/7] kvmppc: Driver to manage pages of secure guest

From: Sukadev Bhattiprolu <[email-masked]>
Date: Thu, 29 Aug 2019 12:39:11 -0700

Bharata B Rao [[email-masked]] wrote:
> On Wed, Aug 28, 2019 at 08:02:19PM -0700, Sukadev Bhattiprolu wrote:
> > Some minor comments/questions below. Overall, the patches look
> > fine to me.
> > 
> > > +#include <linux/pagemap.h>
> > > +#include <linux/migrate.h>
> > > +#include <linux/kvm_host.h>
> > > +#include <asm/ultravisor.h>
> > > +
> > > +static struct dev_pagemap kvmppc_devm_pgmap;
> > > +static unsigned long *kvmppc_devm_pfn_bitmap;
> > > +static DEFINE_SPINLOCK(kvmppc_devm_pfn_lock);
> > 
> > Is this lock protecting just the pfn_bitmap?
> 
> Yes.
> 
> > 
> > > +
> > > +struct kvmppc_devm_page_pvt {
> > > +	unsigned long *rmap;
> > > +	unsigned int lpid;
> > > +	unsigned long gpa;
> > > +};
> > > +
> > > +/*
> > > + * Get a free device PFN from the pool
> > > + *
> > > + * Called when a normal page is moved to secure memory (UV_PAGE_IN). Device
> > > + * PFN will be used to keep track of the secure page on HV side.
> > > + *
> > > + * @rmap here is the slot in the rmap array that corresponds to @gpa.
> > > + * Thus a non-zero rmap entry indicates that the corresponding guest
> > > + * page has become secure, and is not mapped on the HV side.
> > > + *
> > > + * NOTE: In this and subsequent functions, we pass around and access
> > > + * individual elements of kvm_memory_slot->arch.rmap[] without any
> > > + * protection. Should we use lock_rmap() here?

Where do we serialize two threads attempting to H_SVM_PAGE_IN the same gfn
at the same time? Or one thread issuing a H_SVM_PAGE_IN and another a
H_SVM_PAGE_OUT for the same page?

> > > + */
> > > +static struct page *kvmppc_devm_get_page(unsigned long *rmap, unsigned long gpa,
> > > +					 unsigned int lpid)
> > > +{
> > > +	struct page *dpage = NULL;
> > > +	unsigned long bit, devm_pfn;
> > > +	unsigned long flags;
> > > +	struct kvmppc_devm_page_pvt *pvt;
> > > +	unsigned long pfn_last, pfn_first;
> > > +
> > > +	if (kvmppc_rmap_is_devm_pfn(*rmap))
> > > +		return NULL;
> > > +
> > > +	pfn_first = kvmppc_devm_pgmap.res.start >> PAGE_SHIFT;
> > > +	pfn_last = pfn_first +
> > > +		   (resource_size(&kvmppc_devm_pgmap.res) >> PAGE_SHIFT);
> > > +	spin_lock_irqsave(&kvmppc_devm_pfn_lock, flags);
> > 
> > Blank lines around spin_lock() would help.
> 
> You mean blank line before lock and after unlock to clearly see
> where the lock starts and ends?
> 
> > 
> > > +	bit = find_first_zero_bit(kvmppc_devm_pfn_bitmap, pfn_last - pfn_first);
> > > +	if (bit >= (pfn_last - pfn_first))
> > > +		goto out;
> > > +
> > > +	bitmap_set(kvmppc_devm_pfn_bitmap, bit, 1);
> > > +	devm_pfn = bit + pfn_first;
> > 
> > Can we drop the &kvmppc_devm_pfn_lock here or after the trylock_page()?
> > Or does it also protect the ->zone_device_data' assignment below as well?
> > If so, maybe drop the 'pfn_' from the name of the lock?
> > 
> > Besides, we don't seem to hold this lock when accessing ->zone_device_data
> > in kvmppc_share_page(). Maybe &kvmppc_devm_pfn_lock just protects the bitmap?
> 
> Will move the unlock to appropriately.
> 
> > 
> > 
> > > +	dpage = pfn_to_page(devm_pfn);
> > 
> > Does this code and hence CONFIG_PPC_UV depend on a specific model like
> > CONFIG_SPARSEMEM_VMEMMAP?
> 
> I don't think so. Irrespective of that pfn_to_page() should just work
> for us.
> 
> > > +
> > > +	if (!trylock_page(dpage))
> > > +		goto out_clear;
> > > +
> > > +	*rmap = devm_pfn | KVMPPC_RMAP_DEVM_PFN;
> > > +	pvt = kzalloc(sizeof(*pvt), GFP_ATOMIC);
> > > +	if (!pvt)
> > > +		goto out_unlock;

If we fail to alloc, we don't clear the KVMPPC_RMAP_DEVM_PFN?

Also, when/where do we clear this flag on an uv-page-out?
kvmppc_devm_drop_pages() drops the flag on a local variable but not
in the rmap? If we don't clear the flag on page-out, would the
subsequent H_SVM_PAGE_IN of this page fail?

> > > +	pvt->rmap = rmap;
> > > +	pvt->gpa = gpa;
> > > +	pvt->lpid = lpid;
> > > +	dpage->zone_device_data = pvt;
> > 
> > ->zone_device_data is set after locking the dpage here, but in
> > kvmppc_share_page() and kvmppc_devm_fault_migrate_alloc_and_copy()
> > it is accessed without locking the page?
> > 
> > > +	spin_unlock_irqrestore(&kvmppc_devm_pfn_lock, flags);
> > > +
> > > +	get_page(dpage);
> > > +	return dpage;
> > > +
> > > +out_unlock:
> > > +	unlock_page(dpage);
> > > +out_clear:
> > > +	bitmap_clear(kvmppc_devm_pfn_bitmap, devm_pfn - pfn_first, 1);
> > > +out:
> > > +	spin_unlock_irqrestore(&kvmppc_devm_pfn_lock, flags);
> > > +	return NULL;
> > > +}
> > > +
> > > +/*
> > > + * Alloc a PFN from private device memory pool and copy page from normal
> > > + * memory to secure memory.
> > > + */
> > > +static int
> > > +kvmppc_devm_migrate_alloc_and_copy(struct migrate_vma *mig,
> > > +				   unsigned long *rmap, unsigned long gpa,
> > > +				   unsigned int lpid, unsigned long page_shift)
> > > +{
> > > +	struct page *spage = migrate_pfn_to_page(*mig->src);
> > > +	unsigned long pfn = *mig->src >> MIGRATE_PFN_SHIFT;
> > > +	struct page *dpage;
> > > +
> > > +	*mig->dst = 0;
> > > +	if (!spage || !(*mig->src & MIGRATE_PFN_MIGRATE))
> > > +		return 0;
> > > +
> > > +	dpage = kvmppc_devm_get_page(rmap, gpa, lpid);
> > > +	if (!dpage)
> > > +		return -EINVAL;
> > > +
> > > +	if (spage)
> > > +		uv_page_in(lpid, pfn << page_shift, gpa, 0, page_shift);
> > > +
> > > +	*mig->dst = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
> > > +	return 0;
> > > +}
> > > +
> > > +/*
> > > + * Move page from normal memory to secure memory.
> > > + */
> > > +unsigned long
> > > +kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
> > > +		     unsigned long flags, unsigned long page_shift)
> > > +{
> > > +	unsigned long addr, end;
> > > +	unsigned long src_pfn, dst_pfn;
> > 
> > These are the host frame numbers correct? Trying to distinguish them
> > from 'gfn' and 'gpa' used in the function.
> 
> Yes host pfns.
> 
> > 
> > > +	struct migrate_vma mig;
> > > +	struct vm_area_struct *vma;
> > > +	int srcu_idx;
> > > +	unsigned long gfn = gpa >> page_shift;
> > > +	struct kvm_memory_slot *slot;
> > > +	unsigned long *rmap;
> > > +	int ret;
> > > +
> > > +	if (page_shift != PAGE_SHIFT)
> > > +		return H_P3;
> > > +
> > > +	if (flags)
> > > +		return H_P2;
> > > +
> > > +	ret = H_PARAMETER;
> > > +	down_read(&kvm->mm->mmap_sem);
> > > +	srcu_idx = srcu_read_lock(&kvm->srcu);
> > > +	slot = gfn_to_memslot(kvm, gfn);
> > 
> > Can slot be NULL? could be a bug in UV...
> 
> Will add a check to test this failure.
> 
> > 
> > > +	rmap = &slot->arch.rmap[gfn - slot->base_gfn];
> > > +	addr = gfn_to_hva(kvm, gpa >> page_shift);
> > 
> > Use 'gfn' as the second parameter? 
> 
> Yes.
> 
> > 
> > Nit. for consistency with gpa and gfn, maybe rename 'addr' to
> > 'hva' or to match 'end' maybe to 'start'.
> 
> Guess using hva improves readability, sure.
> 
> > 
> > Also, can we check 'kvmppc_rmap_is_devm_pfn(*rmap)' here and bail out
> > if its already shared? We currently do it further down the call chain
> > in kvmppc_devm_get_page() after doing more work.
> 
> If the page is already shared, we just give the same back to UV if
> UV indeed asks for it to be re-shared.
> 
> That said, I think we can have kvmppc_rmap_is_devm_pfn early in
> regular page-in (non-shared case) path so that we don't even setup
> anything required for migrate_vma_pages.
> 
> > 
> > 
> > > +	if (kvm_is_error_hva(addr))
> > > +		goto out;
> > > +
> > > +	end = addr + (1UL << page_shift);
> > > +	vma = find_vma_intersection(kvm->mm, addr, end);
> > > +	if (!vma || vma->vm_start > addr || vma->vm_end < end)
> > > +		goto out;
> > > +
> > > +	memset(&mig, 0, sizeof(mig));
> > > +	mig.vma = vma;
> > > +	mig.start = addr;
> > > +	mig.end = end;
> > > +	mig.src = &src_pfn;
> > > +	mig.dst = &dst_pfn;
> > > +
> > > +	if (migrate_vma_setup(&mig))
> > > +		goto out;
> > > +
> > > +	if (kvmppc_devm_migrate_alloc_and_copy(&mig, rmap, gpa,
> > > +					       kvm->arch.lpid, page_shift))
> > > +		goto out_finalize;
> > > +
> > > +	migrate_vma_pages(&mig);
> > > +	ret = H_SUCCESS;
> > > +out_finalize:
> > > +	migrate_vma_finalize(&mig);
> > > +out:
> > > +	srcu_read_unlock(&kvm->srcu, srcu_idx);
> > > +	up_read(&kvm->mm->mmap_sem);
> > > +	return ret;
> > > +}
> > > +
> > > +/*
> > > + * Provision a new page on HV side and copy over the contents
> > > + * from secure memory.
> > > + */
> > > +static int
> > > +kvmppc_devm_fault_migrate_alloc_and_copy(struct migrate_vma *mig,
> > > +					 unsigned long page_shift)
> > > +{
> > > +	struct page *dpage, *spage;
> > > +	struct kvmppc_devm_page_pvt *pvt;
> > > +	unsigned long pfn;
> > > +	int ret;
> > > +
> > > +	spage = migrate_pfn_to_page(*mig->src);
> > > +	if (!spage || !(*mig->src & MIGRATE_PFN_MIGRATE))
> > > +		return 0;
> > > +	if (!is_zone_device_page(spage))
> > > +		return 0;
> > 
> > What does it mean if its not a zone_device page at this point? Caller
> > would then proceed to migrage_vma_pages() if we return 0 right?
> 
> kvmppc_devm_fault_migrate_alloc_and_copy() can be called from two paths:
> 
> 1. Fault path when HV touches the secure page. In this case the page
> has to be a device page.
> 
> 2. When page-out is issued for a page that is already paged-in. In this
> case also it has be a device page.
> 
> For both the above cases, that check is redundant.
> 
> There is a 3rd case which is possible. If UV ever issues a page-out
> for a shared page, this check will result in page-out hcall silently
> succeeding w/o doing any migration (as we don't populate the dst_pfn)

Ok. Nit. thought we can drop the "_fault" in the function name but would
collide the other "alloc_and_copy" function used during H_SVM_PAGE_IN.
If the two alloc_and_copy functions are symmetric, maybe they could
have "page_in" and "page_out" in the (already long) names.

> 
> > 
> > > +
> > > +	dpage = alloc_page_vma(GFP_HIGHUSER, mig->vma, mig->start);
> > > +	if (!dpage)
> > > +		return -EINVAL;
> > > +	lock_page(dpage);
> > > +	pvt = spage->zone_device_data;
> > > +
> > > +	pfn = page_to_pfn(dpage);
> > > +	ret = uv_page_out(pvt->lpid, pfn << page_shift, pvt->gpa, 0,
> > > +			  page_shift);
> > > +	if (ret == U_SUCCESS)
> > > +		*mig->dst = migrate_pfn(pfn) | MIGRATE_PFN_LOCKED;
> > > +	else {
> > > +		unlock_page(dpage);
> > > +		__free_page(dpage);
> > > +	}
> > > +	return ret;
> > > +}
> > > +
> > > +/*
> > > + * Fault handler callback when HV touches any page that has been
> > > + * moved to secure memory, we ask UV to give back the page by
> > > + * issuing a UV_PAGE_OUT uvcall.
> > > + *
> > > + * This eventually results in dropping of device PFN and the newly
> > > + * provisioned page/PFN gets populated in QEMU page tables.
> > > + */
> > > +static vm_fault_t kvmppc_devm_migrate_to_ram(struct vm_fault *vmf)
> > > +{
> > > +	unsigned long src_pfn, dst_pfn = 0;
> > > +	struct migrate_vma mig;
> > > +	int ret = 0;
> > > +
> > > +	memset(&mig, 0, sizeof(mig));
> > > +	mig.vma = vmf->vma;
> > > +	mig.start = vmf->address;
> > > +	mig.end = vmf->address + PAGE_SIZE;
> > > +	mig.src = &src_pfn;
> > > +	mig.dst = &dst_pfn;
> > > +
> > > +	if (migrate_vma_setup(&mig)) {
> > > +		ret = VM_FAULT_SIGBUS;
> > > +		goto out;
> > > +	}
> > > +
> > > +	if (kvmppc_devm_fault_migrate_alloc_and_copy(&mig, PAGE_SHIFT)) {
> > > +		ret = VM_FAULT_SIGBUS;
> > > +		goto out_finalize;
> > > +	}
> > > +
> > > +	migrate_vma_pages(&mig);
> > > +out_finalize:
> > > +	migrate_vma_finalize(&mig);
> > > +out:
> > > +	return ret;
> > > +}
> > > +
> > > +/*
> > > + * Release the device PFN back to the pool
> > > + *
> > > + * Gets called when secure page becomes a normal page during UV_PAGE_OUT.
> > 
> > Nit: Should that be H_SVM_PAGE_OUT?
> 
> Yes, will reword.
> 
> > 
> > > + */
> > > +static void kvmppc_devm_page_free(struct page *page)
> > > +{
> > > +	unsigned long pfn = page_to_pfn(page);
> > > +	unsigned long flags;
> > > +	struct kvmppc_devm_page_pvt *pvt;
> > > +
> > > +	spin_lock_irqsave(&kvmppc_devm_pfn_lock, flags);
> > > +	pvt = page->zone_device_data;
> > > +	page->zone_device_data = NULL;
> > 
> > If the pfn_lock only protects the bitmap, would be better to move
> > it here?
> 
> Yes.
> 
> > 
> > > +
> > > +	bitmap_clear(kvmppc_devm_pfn_bitmap,
> > > +		     pfn - (kvmppc_devm_pgmap.res.start >> PAGE_SHIFT), 1);
> > > +	*pvt->rmap = 0;
> > > +	spin_unlock_irqrestore(&kvmppc_devm_pfn_lock, flags);
> > > +	kfree(pvt);
> > > +}
> > > +
> > > +static const struct dev_pagemap_ops kvmppc_devm_ops = {
> > > +	.page_free = kvmppc_devm_page_free,
> > > +	.migrate_to_ram	= kvmppc_devm_migrate_to_ram,
> > > +};
> > > +
> > > +/*
> > > + * Move page from secure memory to normal memory.
> > > + */
> > > +unsigned long
> > > +kvmppc_h_svm_page_out(struct kvm *kvm, unsigned long gpa,
> > > +		      unsigned long flags, unsigned long page_shift)
> > > +{
> > > +	struct migrate_vma mig;
> > > +	unsigned long addr, end;
> > > +	struct vm_area_struct *vma;
> > > +	unsigned long src_pfn, dst_pfn = 0;
> > > +	int srcu_idx;
> > > +	int ret;
> > 
> > Nit: Not sure its a coding style requirement, but many functions seem
> > to "sort" these local variables in descending order of line length for
> > appearance :-)  (eg: migrate_vma* functions).
> 
> It has ended up like this over multiple versions when variables got added,
> moved and re-added.
> 
> > 
> > > +
> > > +	if (page_shift != PAGE_SHIFT)
> > > +		return H_P3;
> > > +
> > > +	if (flags)
> > > +		return H_P2;
> > > +
> > > +	ret = H_PARAMETER;
> > > +	down_read(&kvm->mm->mmap_sem);
> > > +	srcu_idx = srcu_read_lock(&kvm->srcu);
> > > +	addr = gfn_to_hva(kvm, gpa >> page_shift);
> > > +	if (kvm_is_error_hva(addr))
> > > +		goto out;
> > > +
> > > +	end = addr + (1UL << page_shift);
> > > +	vma = find_vma_intersection(kvm->mm, addr, end);
> > > +	if (!vma || vma->vm_start > addr || vma->vm_end < end)
> > > +		goto out;
> > > +
> > > +	memset(&mig, 0, sizeof(mig));
> > > +	mig.vma = vma;
> > > +	mig.start = addr;
> > > +	mig.end = end;
> > > +	mig.src = &src_pfn;
> > > +	mig.dst = &dst_pfn;
> > > +	if (migrate_vma_setup(&mig))
> > > +		goto out;
> > > +
> > > +	ret = kvmppc_devm_fault_migrate_alloc_and_copy(&mig, page_shift);
> > > +	if (ret)
> > > +		goto out_finalize;
> > > +
> > > +	migrate_vma_pages(&mig);
> > > +	ret = H_SUCCESS;
> > 
> > Nit: Blank line here?
> 
> With a blank like above the label line (which is blank for the most part),
> it looks a bit too much of blank to me :)
> 
> However I do have blank line at a few other places. I have been removing
> them whenever I touch the surrounding lines.
> 
> Thanks for your review.
> 
> Christoph - You did review this patch in the last iteration. Do you have
> any additional comments?
> 
> Regards,
> Bharata.