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

[Sean Christopherson]

On Wed, Sep 02, 2020 at 04:12:55PM +0200, Vitaly Kuznetsov wrote:
> Lai Jiangshan <jiangshanlai@xxxxxxxxx> writes:
> 
> > 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 as above for many years, But
> > 8c8560b83390("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes)
> 
> Nit: checkpatch.pl complains here with 
> 
> ERROR: Please use git commit description style 'commit <12+ chars of
> sha1> ("<title line>")' - ie: 'commit 8c8560b83390 ("KVM: x86/mmu: Use
> KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes")'
> #118: 
> 8c8560b83390("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes)

Definitely needs a

  Fixes: 8c8560b83390("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes)

At that point I'd just have the changelog say "a recent commit".

> > 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@xxxxxxxxx/
> > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> > 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);
> 
> FWIW, 
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> 
> but it'd be great to hear from Sean).

I got nothing, AFAICT I was simply overzealous.

Reviewed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>