Re: [PATCH V3] kvm x86/mmu: use KVM_REQ_MMU_SYNC to sync when needed

[Lai Jiangshan]

Hi, Paolo

Could you pick the patch please?

I think it'd be better to be merged before v5.9 is released
since it fixes a flaw.

Thanks
Lai

On Thu, Sep 3, 2020 at 11:22 PM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
>
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
>
> When kvm_mmu_get_page() gets a page with unsynced children, the spt
> pagetable is unsynchronized with the guest pagetable. But the
> guest might not issue a "flush" operation on it when the pagetable
> entry is changed from zero or other cases. The hypervisor has the
> responsibility to synchronize the pagetables.
>
> The linux kernel behaves correctly as above for many years, but a recent
> commit 8c8560b83390 ("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for
> MMU specific flushes") inadvertently included a line of code to change it
> without giving any reason in the changelog. It is clear that the commit's
> intention was to change KVM_REQ_TLB_FLUSH -> KVM_REQ_TLB_FLUSH_CURRENT,
> so we don't unneedlesly flush other contexts but one of the hunks changed
> nearby KVM_REQ_MMU_SYNC instead.
>
> This patch changes it back.
>
> Fixes: 8c8560b83390("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes)
> Link: https://lore.kernel.org/lkml/20200320212833.3507-26-sean.j.christopherson@xxxxxxxxx/
> Reviewed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---
> Changed from v1:
>         update patch description
>
> Changed form v2:
>         update patch description
>
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4e03841f053d..9a93de921f2b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2468,7 +2468,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>                 }
>
>                 if (sp->unsync_children)
> -                       kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> +                       kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
>
>                 __clear_sp_write_flooding_count(sp);
>
> --
> 2.19.1.6.gb485710b
>