On Tue, Dec 04, 2012 at 07:17:11AM +0800, Xiao Guangrong wrote:
> There are two cases we need to adjust page size in set_spte:
> 1): the one is other vcpu creates new sp in the window between mapping_level()
> and acquiring mmu-lock.
> 2): the another case is the new sp is created by itself (page-fault path) when
> guest uses the target gfn as its page table.
>
> In current code, set_spte drop the spte and emulate the access for these case,
> it works not good:
> - for the case 1, it may destroy the mapping established by other vcpu, and
> do expensive instruction emulation.
> - for the case 2, it may emulate the access even if the guest is accessing
> the page which not used as page table. There is a example, 0~2M is used as
> huge page in guest, in this huge page, only page 3 used as page table, then
> guest read/writes on other pages can cause instruction emulation.
>
> Both of these cases can be fixed by allowing guest to retry the access, it
> will refault, then we can establish the mapping by using small page
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
Applied to queue. Thanks.
> ---
> arch/x86/kvm/mmu.c | 16 ++++++++++++----
> 1 files changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index b875a9e..01d7c2a 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2382,12 +2382,20 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> || (!vcpu->arch.mmu.direct_map && write_fault
> && !is_write_protection(vcpu) && !user_fault)) {
>
> + /*
> + * There are two cases:
> + * - the one is other vcpu creates new sp in the window
> + * between mapping_level() and acquiring mmu-lock.
> + * - the another case is the new sp is created by itself
> + * (page-fault path) when guest uses the target gfn as
> + * its page table.
> + * Both of these cases can be fixed by allowing guest to
> + * retry the access, it will refault, then we can establish
> + * the mapping by using small page.
> + */
> if (level > PT_PAGE_TABLE_LEVEL &&
> - has_wrprotected_page(vcpu->kvm, gfn, level)) {
> - ret = 1;
> - drop_spte(vcpu->kvm, sptep);
> + has_wrprotected_page(vcpu->kvm, gfn, level))
> goto done;
> - }
>
> spte |= PT_WRITABLE_MASK | SPTE_MMU_WRITEABLE;
>
> --
> 1.7.7.6
--
Gleb.
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