linux-kernel - Re: [PATCH] kvm: x86/mmu: Remove FNAME(is_self_change

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Date:   Wed, 1 Feb 2023 19:19:59 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org
Subject: Re: [PATCH] kvm: x86/mmu: Remove FNAME(is_self_change_mapping)

On Tue, Dec 13, 2022, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@...group.com>
> 
> FNAME(is_self_change_mapping) has two functionalities.
> 
>   If the fault is on a huge page but at least one of the pagetable on
>   the walk is also on the terminal huge page, disable the huge page
>   mapping for the fault.
> 
>   If the fault is modifying at least one of the pagetable on the walk,
>   set something to tell the emulator.

This should be two patches, one to move the arch.write_fault_to_shadow_pgtable
handling and one to drop the hugepage adjustment.

I also want to rework the handling of write_fault_to_shadow_pgtable as prep work.
Every time I look at that flag it takes me an eternity to remember exactly how
KVM guarantees x86_emulate_instruction() won't get false positives.  I.e. I always
forget why it's ok to not clear vcpu->arch.write_fault_to_shadow_pgtable after
every VM-Exit.

Unless I've missed something, we can use an EMULTYPE flag to communicate to the
emulator that the #PF emulation is on a self-referential write to a shadow page.
That allows dropping write_fault_to_shadow_pgtable from vcpu->arch and sidesteps
the whole "how do we avoid false positives?" question.

Testing now, if everything looks good, I'll post v2 with all three patches.