Re: [PATCH V2] kvm x86/mmu: use KVM_REQ_MMU_SYNC to sync when needed

Paolo Bonzini
2020-09-11T17:17:39Z

On 02/09/20 15:54, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> 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 as above for many years, But
> 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.
>
> The this patch changes it back.
>
> Link: https://lore.kernel.org/lkml/20200320212833.3507-26-sean.j.christopherson@intel.com/
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> Changed from v1:
> 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);
>
>

Queued, thanks.

Paolo