linux-kernel - Re: [PATCH 1/3] KVM: x86/mmu: Tweak PSE hugepage handling to avoid 2M vs 4M conundrum

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Date:   Tue, 28 Apr 2020 10:07:27 -0400
From:   Barret Rhoden <brho@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] KVM: x86/mmu: Tweak PSE hugepage handling to avoid 2M
 vs 4M conundrum

On 4/27/20 8:54 PM, Sean Christopherson wrote:
> Change the PSE hugepage handling in walk_addr_generic() to fire on any
> page level greater than PT_PAGE_TABLE_LEVEL, a.k.a. PG_LEVEL_4K.  PSE
> paging only has two levels, so "== 2" and "> 1" are functionally the
> seam, i.e. this is a nop.
   ^ s/seam/same/

Barret

> 
> A future patch will drop KVM's PT_*_LEVEL enums in favor of the kernel's
> PG_LEVEL_* enums, at which point "walker->level == PG_LEVEL_2M" is
> semantically incorrect (though still functionally ok).
> 
> No functional change intended.
> 
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>   arch/x86/kvm/mmu/paging_tmpl.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index efec7d27b8c5..ca39bd315f70 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -436,7 +436,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
>   	gfn = gpte_to_gfn_lvl(pte, walker->level);
>   	gfn += (addr & PT_LVL_OFFSET_MASK(walker->level)) >> PAGE_SHIFT;
>   
> -	if (PTTYPE == 32 && walker->level == PT_DIRECTORY_LEVEL && is_cpuid_PSE36())
> +	if (PTTYPE == 32 && walker->level > PT_PAGE_TABLE_LEVEL && is_cpuid_PSE36())
>   		gfn += pse36_gfn_delta(pte);
>   
>   	real_gpa = mmu->translate_gpa(vcpu, gfn_to_gpa(gfn), access, &walker->fault);
>