Subject: Re: [PATCH 06/11] powerpc/kvm: Allow KVM_PPC_ALLOCATE_HTAB ioctl() to change HPT size

From: David Gibson <[email-masked]>
Date: Mon, 19 Dec 2016 11:48:09 +1100

On Fri, Dec 16, 2016 at 01:44:57PM +0100, Thomas Huth wrote:
> On 15.12.2016 06:53, David Gibson wrote:
> > The KVM_PPC_ALLOCATE_HTAB ioctl() is used to set the size of hashed page
> > table (HPT) that userspace expects a guest VM to have, and is also used to
> > clear that HPT when necessary (e.g. guest reboot).
> > 
> > At present, once the ioctl() is called for the first time, the HPT size can
> > never be changed thereafter - it will be cleared but always sized as from
> > the first call.
> > 
> > With upcoming HPT resize implementation, we're going to need to allow
> > userspace to resize the HPT at reset (to change it back to the default size
> > if the guest changed it).
> > 
> > So, we need to allow this ioctl() to change the HPT size.
> > 
> > Signed-off-by: David Gibson <[email-masked]>
> > ---
> >  arch/powerpc/include/asm/kvm_ppc.h  |  2 +-
> >  arch/powerpc/kvm/book3s_64_mmu_hv.c | 53 ++++++++++++++++++++-----------------
> >  arch/powerpc/kvm/book3s_hv.c        |  5 +---
> >  3 files changed, 30 insertions(+), 30 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
> > index 41575b8..3b837bc 100644
> > --- a/arch/powerpc/include/asm/kvm_ppc.h
> > +++ b/arch/powerpc/include/asm/kvm_ppc.h
> > @@ -157,7 +157,7 @@ extern void kvmppc_map_magic(struct kvm_vcpu *vcpu);
> >  
> >  extern int kvmppc_allocate_hpt(struct kvm_hpt_info *info, u32 order);
> >  extern void kvmppc_set_hpt(struct kvm *kvm, struct kvm_hpt_info *info);
> > -extern long kvmppc_alloc_reset_hpt(struct kvm *kvm, u32 *htab_orderp);
> > +extern long kvmppc_alloc_reset_hpt(struct kvm *kvm, int order);
> >  extern void kvmppc_free_hpt(struct kvm_hpt_info *info);
> >  extern long kvmppc_prepare_vrma(struct kvm *kvm,
> >  				struct kvm_userspace_memory_region *mem);
> > diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> > index 68bb228..8e5ac2f 100644
> > --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> > +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> > @@ -104,10 +104,22 @@ void kvmppc_set_hpt(struct kvm *kvm, struct kvm_hpt_info *info)
> >  		info->virt, (long)info->order, kvm->arch.lpid);
> >  }
> >  
> > -long kvmppc_alloc_reset_hpt(struct kvm *kvm, u32 *htab_orderp)
> > +void kvmppc_free_hpt(struct kvm_hpt_info *info)
> > +{
> > +	vfree(info->rev);
> > +	if (info->cma)
> > +		kvm_free_hpt_cma(virt_to_page(info->virt),
> > +				 1 << (info->order - PAGE_SHIFT));
> > +	else
> > +		free_pages(info->virt, info->order - PAGE_SHIFT);
> > +	info->virt = 0;
> > +	info->order = 0;
> > +}
> 
> Why do you need to move kvmppc_free_hpt() around? Seems like unecessary
> code churn to me?

Previously, kvmppc_free_hpt() wasn't needed in
kvmppc_alloc_reset_hpt(), now it is.  So we need to move it above that
function.  I could move it in the previous patch, but that would
obscure what the actual changes are to it, so it seemed better to do
it here.

> 
> > +long kvmppc_alloc_reset_hpt(struct kvm *kvm, int order)
> >  {
> >  	long err = -EBUSY;
> > -	long order;
> > +	struct kvm_hpt_info info;
> >  
> >  	mutex_lock(&kvm->lock);
> >  	if (kvm->arch.hpte_setup_done) {
> > @@ -119,8 +131,9 @@ long kvmppc_alloc_reset_hpt(struct kvm *kvm, u32 *htab_orderp)
> >  			goto out;
> >  		}
> >  	}
> > -	if (kvm->arch.hpt.virt) {
> > -		order = kvm->arch.hpt.order;
> > +	if (kvm->arch.hpt.order == order) {
> > +		/* We already have a suitable HPT */
> > +
> >  		/* Set the entire HPT to 0, i.e. invalid HPTEs */
> >  		memset((void *)kvm->arch.hpt.virt, 0, 1ul << order);
> >  		/*
> > @@ -129,33 +142,23 @@ long kvmppc_alloc_reset_hpt(struct kvm *kvm, u32 *htab_orderp)
> >  		kvmppc_rmap_reset(kvm);
> >  		/* Ensure that each vcpu will flush its TLB on next entry. */
> >  		cpumask_setall(&kvm->arch.need_tlb_flush);
> > -		*htab_orderp = order;
> >  		err = 0;
> > -	} else {
> > -		struct kvm_hpt_info info;
> > -
> > -		err = kvmppc_allocate_hpt(&info, *htab_orderp);
> > -		if (err < 0)
> > -			goto out;
> > -		kvmppc_set_hpt(kvm, &info);
> > +		goto out;
> >  	}
> > - out:
> > +
> > +	if (kvm->arch.hpt.virt)
> > +		kvmppc_free_hpt(&kvm->arch.hpt);
> > +
> > +	err = kvmppc_allocate_hpt(&info, order);
> > +	if (err < 0)
> > +		goto out;
> > +	kvmppc_set_hpt(kvm, &info);
> > +
> > +out:
> >  	mutex_unlock(&kvm->lock);
> >  	return err;
> >  }
> >  
> > -void kvmppc_free_hpt(struct kvm_hpt_info *info)
> > -{
> > -	vfree(info->rev);
> > -	if (info->cma)
> > -		kvm_free_hpt_cma(virt_to_page(info->virt),
> > -				 1 << (info->order - PAGE_SHIFT));
> > -	else
> > -		free_pages(info->virt, info->order - PAGE_SHIFT);
> > -	info->virt = 0;
> > -	info->order = 0;
> > -}
> > -
> >  /* Bits in first HPTE dword for pagesize 4k, 64k or 16M */
> >  static inline unsigned long hpte0_pgsize_encoding(unsigned long pgsize)
> >  {
> > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > index 71c5adb..957e473 100644
> > --- a/arch/powerpc/kvm/book3s_hv.c
> > +++ b/arch/powerpc/kvm/book3s_hv.c
> > @@ -3600,12 +3600,9 @@ static long kvm_arch_vm_ioctl_hv(struct file *filp,
> >  		r = -EFAULT;
> >  		if (get_user(htab_order, (u32 __user *)argp))
> >  			break;
> > -		r = kvmppc_alloc_reset_hpt(kvm, &htab_order);
> > +		r = kvmppc_alloc_reset_hpt(kvm, htab_order);
> >  		if (r)
> >  			break;
> > -		r = -EFAULT;
> > -		if (put_user(htab_order, (u32 __user *)argp))
> > -			break;
> 
> Now that htab_order is not changed anymore by the kernel, I'm pretty
> sure you need some checks on the value here before calling
> kvmppc_alloc_reset_hpt(), e.g. return an error code if htab_order <
> PPC_MIN_HPT_ORDER.

Right.  I've done that by putting the checks into
kvmppc_allocate_hpt() in the earlier patch.

> And, I'm not sure if I got that right, but in former times, the
> htab_order from the userspace application was just a suggestion, and now
> it's mandatory, right? So if an old userspace used a very high value
> here (or even something smaller than PPC_MIN_HPT_ORDER like 0), the
> kernel fixed this up and the userspace could run happily with the fixed
> value afterwards. But since this value from userspace if mandatory now,
> such an userspace application is broken now. So maybe it's better to
> introduce a new ioctl for this new behavior instead, to avoid breaking
> old userspace applications?

A long time ago it was just a hint.  However, that behaviour was
already changed in 572abd5 "KVM: PPC: Book3S HV: Don't fall back to
smaller HPT size in allocation ioctl".  This is important: without
that we could get a different HPT size on the two ends of a migration,
which broke things nastily.

> Anyway, you should also update Documentation/virtual/kvm/api.txt to
> reflect the new behavior.

Good point.  Done.

> >  		r = 0;
> >  		break;
> >  	}
>  Thomas

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson