linux-kernel - [PATCH] KVM: use slowpath for cross page cached accesses

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Date:	Wed, 8 Apr 2015 14:16:48 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: [PATCH] KVM: use slowpath for cross page cached accesses

2015-04-08 12:43+0200, Paolo Bonzini:
> On 08/04/2015 11:26, Radim Krčmář wrote:
>> 2015-04-08 10:49+0200, Paolo Bonzini:
>>> On 07/04/2015 22:34, Radim Krčmář wrote:
>>>> We dirtied only one page because writes originally couldn't span more.
>>>> Use improved syntax for '>> PAGE_SHIFT' while at it.
>>>>
>>>> Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.")
>>>> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
>>>
>>> Cross-page reads and writes should never get here; they have
>>> ghc->memslot set to NULL and go through the slow path in kvm_write_guest.
>> 
>> Only cross-memslot writes have NULL memslot.
> 
> The power of wrong comments...
> 
> Considering how kvm_gfn_to_hva_cache_init is used (one 1-byte field, two
> 4-byte fields, one 28-bytes struct that is 32-bytes aligned, one
> 32-bytes field that is in practice cacheline-aligned), I wonder if we
> should just use ghc->memslot = NULL for cross page writes.  This would
> bypass the bug you are fixing here, and avoid worries about partial writes.

Good idea, and it could make those comments right :)
(Though in general, I prefer less constraints on APIs ...)

Partial writes would be a pain;  copy_to_user API does not define which
bytes were not written.  I think the write can't fail mid-page, which
makes our implementation ok, but I still worry a bit about it.

Anyway, here's the patch:

---8<---
kvm_write_guest_cached() does not mark all written pages as dirty and
code comments in kvm_gfn_to_hva_cache_init() talk about NULL memslot
with cross page accesses.  Fix all the easy way.

The check is '<= 1' to have the same result for 'len = 0' cache anywhere
in the page.  (nr_pages_needed is 0 on page boundary.)

Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.")
Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
---
 virt/kvm/kvm_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index aadef264bed1..f3dc641f9640 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1637,8 +1637,8 @@ int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 	ghc->generation = slots->generation;
 	ghc->len = len;
 	ghc->memslot = gfn_to_memslot(kvm, start_gfn);
-	ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, &nr_pages_avail);
-	if (!kvm_is_error_hva(ghc->hva) && nr_pages_avail >= nr_pages_needed) {
+	ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn, NULL);
+	if (!kvm_is_error_hva(ghc->hva) && nr_pages_needed <= 1) {
 		ghc->hva += offset;
 	} else {
 		/*
--
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