Subject: Re: [RFC PATCH v2 12/26] KVM: arm64: Introduce a Hyp buddy page allocator

From: Quentin Perret <[email-masked]>
Date: Wed, 3 Feb 2021 18:33:30 +0000

Hey Will,

On Tuesday 02 Feb 2021 at 18:13:08 (+0000), Will Deacon wrote:
> Hi Quentin,
> 
> Sorry for the delay, this one took me a while to grok.

No need to be sorry, thanks for having a look!

> On Fri, Jan 08, 2021 at 12:15:10PM +0000, Quentin Perret wrote:
> > When memory protection is enabled, the hyp code will require a basic
> > form of memory management in order to allocate and free memory pages at
> > EL2. This is needed for various use-cases, including the creation of hyp
> > mappings or the allocation of stage 2 page tables.
> > 
> > To address these use-case, introduce a simple memory allocator in the
> > hyp code. The allocator is designed as a conventional 'buddy allocator',
> > working with a page granularity. It allows to allocate and free
> > physically contiguous pages from memory 'pools', with a guaranteed order
> > alignment in the PA space. Each page in a memory pool is associated
> > with a struct hyp_page which holds the page's metadata, including its
> > refcount, as well as its current order, hence mimicking the kernel's
> > buddy system in the GFP infrastructure. The hyp_page metadata are made
> > accessible through a hyp_vmemmap, following the concept of
> > SPARSE_VMEMMAP in the kernel.
> > 
> > Signed-off-by: Quentin Perret <[email-masked]>
> > ---
> >  arch/arm64/kvm/hyp/include/nvhe/gfp.h    |  32 ++++
> >  arch/arm64/kvm/hyp/include/nvhe/memory.h |  25 +++
> >  arch/arm64/kvm/hyp/nvhe/Makefile         |   2 +-
> >  arch/arm64/kvm/hyp/nvhe/page_alloc.c     | 185 +++++++++++++++++++++++
> >  4 files changed, 243 insertions(+), 1 deletion(-)
> >  create mode 100644 arch/arm64/kvm/hyp/include/nvhe/gfp.h
> >  create mode 100644 arch/arm64/kvm/hyp/nvhe/page_alloc.c
> > 
> > diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > new file mode 100644
> > index 000000000000..95587faee171
> > --- /dev/null
> > +++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > @@ -0,0 +1,32 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +#ifndef __KVM_HYP_GFP_H
> > +#define __KVM_HYP_GFP_H
> > +
> > +#include <linux/list.h>
> > +
> > +#include <nvhe/memory.h>
> > +#include <nvhe/spinlock.h>
> > +
> > +#define HYP_MAX_ORDER	11U
> 
> Could we just use MAX_ORDER here?

Sure, that would work too. I just figured we might decide to set this to
a lower value in the future -- this effectively limits the size of the
largest portion of memory we can allocate, so maybe it would make sense
to set that to match the size of the largest concatenated pgd for ex,
hence minimizing the overhead of struct hyp_pool. But I suppose we
can also do that later, so...

> > +#define HYP_NO_ORDER	UINT_MAX
> > +
> > +struct hyp_pool {
> > +	hyp_spinlock_t lock;
> 
> A comment about what this lock protects would be handy, especially as the
> 'refcount' field of 'struct hyp_page' isn't updated atomically. I think it
> also means that we don't have a safe way to move a page from one pool to
> another; it's fixed forever once the page has been made available for
> allocation.

Indeed, there is currently no good way to do this. I'll stick a comment.

> > +	struct list_head free_area[HYP_MAX_ORDER + 1];
> > +	phys_addr_t range_start;
> > +	phys_addr_t range_end;
> > +};
> > +
> > +/* GFP flags */
> > +#define HYP_GFP_NONE	0
> > +#define HYP_GFP_ZERO	1
> > +
> > +/* Allocation */
> > +void *hyp_alloc_pages(struct hyp_pool *pool, gfp_t mask, unsigned int order);
> > +void hyp_get_page(void *addr);
> > +void hyp_put_page(void *addr);
> > +
> > +/* Used pages cannot be freed */
> > +int hyp_pool_init(struct hyp_pool *pool, phys_addr_t phys,
> > +		  unsigned int nr_pages, unsigned int used_pages);
> 
> Maybe "reserved_pages" would be a better name than "used_pages"?

That works too. These pages could maybe use a bit of love as well,
they're the pages that have been allocated by the early allocator before
we hand over the memory pool to this allocator. So we might want to do
something about them (such as fixup their refcount).

> > +#endif /* __KVM_HYP_GFP_H */
> > diff --git a/arch/arm64/kvm/hyp/include/nvhe/memory.h b/arch/arm64/kvm/hyp/include/nvhe/memory.h
> > index 64c44c142c95..ed47674bc988 100644
> > --- a/arch/arm64/kvm/hyp/include/nvhe/memory.h
> > +++ b/arch/arm64/kvm/hyp/include/nvhe/memory.h
> > @@ -6,7 +6,17 @@
> >  
> >  #include <linux/types.h>
> >  
> > +struct hyp_pool;
> > +struct hyp_page {
> > +	unsigned int refcount;
> > +	unsigned int order;
> > +	struct hyp_pool *pool;
> > +	struct list_head node;
> > +};
> > +
> >  extern s64 hyp_physvirt_offset;
> > +extern u64 __hyp_vmemmap;
> > +#define hyp_vmemmap ((struct hyp_page *)__hyp_vmemmap)
> >  
> >  #define __hyp_pa(virt)	((phys_addr_t)(virt) + hyp_physvirt_offset)
> >  #define __hyp_va(virt)	((void *)((phys_addr_t)(virt) - hyp_physvirt_offset))
> > @@ -21,4 +31,19 @@ static inline phys_addr_t hyp_virt_to_phys(void *addr)
> >  	return __hyp_pa(addr);
> >  }
> >  
> > +#define hyp_phys_to_pfn(phys)	((phys) >> PAGE_SHIFT)
> > +#define hyp_phys_to_page(phys)	(&hyp_vmemmap[hyp_phys_to_pfn(phys)])
> > +#define hyp_virt_to_page(virt)	hyp_phys_to_page(__hyp_pa(virt))
> > +
> > +#define hyp_page_to_phys(page)  ((phys_addr_t)((page) - hyp_vmemmap) << PAGE_SHIFT)
> 
> Maybe implement this in terms of a new hyp_page_to_pfn() macro?

Sure, should be easy enough.

> > +#define hyp_page_to_virt(page)	__hyp_va(hyp_page_to_phys(page))
> > +#define hyp_page_to_pool(page)	(((struct hyp_page *)page)->pool)
> > +
> > +static inline int hyp_page_count(void *addr)
> > +{
> > +	struct hyp_page *p = hyp_virt_to_page(addr);
> > +
> > +	return p->refcount;
> > +}
> > +
> >  #endif /* __KVM_HYP_MEMORY_H */
> > diff --git a/arch/arm64/kvm/hyp/nvhe/Makefile b/arch/arm64/kvm/hyp/nvhe/Makefile
> > index 33bd381d8f73..9e5eacfec6ec 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/Makefile
> > +++ b/arch/arm64/kvm/hyp/nvhe/Makefile
> > @@ -10,7 +10,7 @@ lib-objs := clear_page.o copy_page.o memcpy.o memset.o
> >  lib-objs := $(addprefix ../../../lib/, $(lib-objs))
> >  
> >  obj-y := timer-sr.o sysreg-sr.o debug-sr.o switch.o tlb.o hyp-init.o host.o \
> > -	 hyp-main.o hyp-smp.o psci-relay.o early_alloc.o stub.o
> > +	 hyp-main.o hyp-smp.o psci-relay.o early_alloc.o stub.o page_alloc.o
> >  obj-y += ../vgic-v3-sr.o ../aarch32.o ../vgic-v2-cpuif-proxy.o ../entry.o \
> >  	 ../fpsimd.o ../hyp-entry.o ../exception.o
> >  obj-y += $(lib-objs)
> > diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> > new file mode 100644
> > index 000000000000..6de6515f0432
> > --- /dev/null
> > +++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> > @@ -0,0 +1,185 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copyright (C) 2020 Google LLC
> > + * Author: Quentin Perret <[email-masked]>
> > + */
> > +
> > +#include <asm/kvm_hyp.h>
> > +#include <nvhe/gfp.h>
> > +
> > +u64 __hyp_vmemmap;
> > +
> > +/*
> > + * Example buddy-tree for a 4-pages physically contiguous pool:
> > + *
> > + *                 o : Page 3
> > + *                /
> > + *               o-o : Page 2
> > + *              /
> > + *             /   o : Page 1
> > + *            /   /
> > + *           o---o-o : Page 0
> > + *    Order  2   1 0
> > + *
> > + * Example of requests on this zon:
> 
> typo: zone

s/zon/pool, even :)

> > + *   __find_buddy(pool, page 0, order 0) => page 1
> > + *   __find_buddy(pool, page 0, order 1) => page 2
> > + *   __find_buddy(pool, page 1, order 0) => page 0
> > + *   __find_buddy(pool, page 2, order 0) => page 3
> > + */
> > +static struct hyp_page *__find_buddy(struct hyp_pool *pool, struct hyp_page *p,
> > +				     unsigned int order)
> > +{
> > +	phys_addr_t addr = hyp_page_to_phys(p);
> > +
> > +	addr ^= (PAGE_SIZE << order);
> > +	if (addr < pool->range_start || addr >= pool->range_end)
> > +		return NULL;
> 
> Are these range checks only needed because the pool isn't required to be
> an exact power-of-2 pages in size? If so, maybe it would be more
> straightforward to limit the max order on a per-pool basis depending upon
> its size?

More importantly, it is because pages outside of the pool are not
guaranteed to be covered by the hyp_vmemmap, so I really need to make
sure I don't dereference them.

> > +
> > +	return hyp_phys_to_page(addr);
> > +}
> > +
> > +static void __hyp_attach_page(struct hyp_pool *pool,
> > +			      struct hyp_page *p)
> > +{
> > +	unsigned int order = p->order;
> > +	struct hyp_page *buddy;
> > +
> > +	p->order = HYP_NO_ORDER;
> 
> Why is this needed?

If p->order is say 3, I may be able to coalesce with the buddy of order
3 to form a higher order page of order 4. And that higher order page
will be represented by the 'first' of the two order-3 pages (let's call
it the head), and the other order 3 page (let's say the tail) will be
assigned 'HYP_NO_ORDER'.

And basically at this point I don't know if 'p' is going be the head or
the tail, so I set it to HYP_NO_ORDER a priori so I don't have to think
about this in the loop below. Is that helping?

I suppose this could use more comments as well ...

> 
> > +	for (; order < HYP_MAX_ORDER; order++) {
> > +		/* Nothing to do if the buddy isn't in a free-list */
> > +		buddy = __find_buddy(pool, p, order);
> > +		if (!buddy || list_empty(&buddy->node) || buddy->order != order)
> 
> Could we move the "buddy->order" check into __find_buddy()?

I think might break __hyp_extract_page() below. The way I think about
__find_buddy() is as a low level function which gives you the buddy page
blindly if it exists in the hyp_vmemmap, and it's up to the callers to
decide whether the buddy is in the right state for their use or not.

Again, a comment would help I guess.

> 
> > +			break;
> > +
> > +		/* Otherwise, coalesce the buddies and go one level up */
> > +		list_del_init(&buddy->node);
> > +		buddy->order = HYP_NO_ORDER;
> > +		p = (p < buddy) ? p : buddy;
> > +	}
> > +
> > +	p->order = order;
> > +	list_add_tail(&p->node, &pool->free_area[order]);
> > +}
> > +
> > +void hyp_put_page(void *addr)
> > +{
> > +	struct hyp_page *p = hyp_virt_to_page(addr);
> > +	struct hyp_pool *pool = hyp_page_to_pool(p);
> > +
> > +	hyp_spin_lock(&pool->lock);
> > +	if (!p->refcount)
> > +		hyp_panic();
> > +	p->refcount--;
> > +	if (!p->refcount)
> > +		__hyp_attach_page(pool, p);
> > +	hyp_spin_unlock(&pool->lock);
> > +}
> > +
> > +void hyp_get_page(void *addr)
> > +{
> > +	struct hyp_page *p = hyp_virt_to_page(addr);
> > +	struct hyp_pool *pool = hyp_page_to_pool(p);
> > +
> > +	hyp_spin_lock(&pool->lock);
> > +	p->refcount++;
> > +	hyp_spin_unlock(&pool->lock);
> 
> We should probably have a proper atomic refcount type for this along the
> lines of refcount_t. Even if initially that is implemented with a lock, it
> would be good to hide that behind a refcount API.

Makes sense, I'll introduce wrappers around these.
> 
> > +}
> > +
> > +/* Extract a page from the buddy tree, at a specific order */
> > +static struct hyp_page *__hyp_extract_page(struct hyp_pool *pool,
> > +					   struct hyp_page *p,
> > +					   unsigned int order)
> > +{
> > +	struct hyp_page *buddy;
> > +
> > +	if (p->order == HYP_NO_ORDER || p->order < order)
> > +		return NULL;
> 
> Can you drop the explicit HYP_NO_ORDER check here?

I think so, yes.

> > +
> > +	list_del_init(&p->node);
> > +
> > +	/* Split the page in two until reaching the requested order */
> > +	while (p->order > order) {
> > +		p->order--;
> > +		buddy = __find_buddy(pool, p, p->order);
> > +		buddy->order = p->order;
> > +		list_add_tail(&buddy->node, &pool->free_area[buddy->order]);
> > +	}
> > +
> > +	p->refcount = 1;
> > +
> > +	return p;
> > +}
> > +
> > +static void clear_hyp_page(struct hyp_page *p)
> > +{
> > +	unsigned long i;
> > +
> > +	for (i = 0; i < (1 << p->order); i++)
> > +		clear_page(hyp_page_to_virt(p) + (i << PAGE_SHIFT));
> 
> I wonder if this is actually any better than a memset(0)? That should use
> DC ZCA as appropriate afaict.

I think that makes sense, and would allow us to drop the EL2 dependency
on clear_page(), so I'll do the change for v3.

> > +static void *__hyp_alloc_pages(struct hyp_pool *pool, gfp_t mask,
> > +			       unsigned int order)
> > +{
> > +	unsigned int i = order;
> > +	struct hyp_page *p;
> > +
> > +	/* Look for a high-enough-order page */
> > +	while (i <= HYP_MAX_ORDER && list_empty(&pool->free_area[i]))
> > +		i++;
> > +	if (i > HYP_MAX_ORDER)
> > +		return NULL;
> > +
> > +	/* Extract it from the tree at the right order */
> > +	p = list_first_entry(&pool->free_area[i], struct hyp_page, node);
> > +	p = __hyp_extract_page(pool, p, order);
> > +
> > +	if (mask & HYP_GFP_ZERO)
> > +		clear_hyp_page(p);
> 
> Do we have a use-case where skipping the zeroing is worthwhile? If not,
> it might make some sense to zero on the freeing path instead.

And during hyp_pool_init(), but this should be infrequent, so yes I
think this is preferable. I'll get rid of HYP_GFP_ZERO altogether.

> 
> > +
> > +	return p;
> > +}
> > +
> > +void *hyp_alloc_pages(struct hyp_pool *pool, gfp_t mask, unsigned int order)
> > +{
> > +	struct hyp_page *p;
> > +
> > +	hyp_spin_lock(&pool->lock);
> > +	p = __hyp_alloc_pages(pool, mask, order);
> > +	hyp_spin_unlock(&pool->lock);
> > +
> > +	return p ? hyp_page_to_virt(p) : NULL;
> 
> It looks weird not having __hyp_alloc_pages return the VA, but I guess later
> patches will use __hyp_alloc_pages() for something else.

Actually no, this can be simplified.

> > +}
> > +
> > +/* hyp_vmemmap must be backed beforehand */
> > +int hyp_pool_init(struct hyp_pool *pool, phys_addr_t phys,
> > +		  unsigned int nr_pages, unsigned int used_pages)
> > +{
> > +	struct hyp_page *p;
> > +	int i;
> > +
> > +	if (phys % PAGE_SIZE)
> > +		return -EINVAL;
> 
> Maybe just take a pfn instead?
> 
> > +	hyp_spin_lock_init(&pool->lock);
> > +	for (i = 0; i <= HYP_MAX_ORDER; i++)
> > +		INIT_LIST_HEAD(&pool->free_area[i]);
> > +	pool->range_start = phys;
> > +	pool->range_end = phys + (nr_pages << PAGE_SHIFT);
> > +
> > +	/* Init the vmemmap portion */
> > +	p = hyp_phys_to_page(phys);
> > +	memset(p, 0, sizeof(*p) * nr_pages);
> > +	for (i = 0; i < nr_pages; i++, p++) {
> > +		p->pool = pool;
> > +		INIT_LIST_HEAD(&p->node);
> > +	}
> 
> Maybe index p like an array (e.g. p[i]) instead of maintaining two loop
> increments?
> 
> > +
> > +	/* Attach the unused pages to the buddy tree */
> > +	p = hyp_phys_to_page(phys + (used_pages << PAGE_SHIFT));
> > +	for (i = used_pages; i < nr_pages; i++, p++)
> > +		__hyp_attach_page(pool, p);
> 
> Likewise.

And ack for these 3 comments.

Cheers,
Quentin

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