linux-kernel - Re: [PATCH V2 4/8] kvm: x86/mmu: Set mmu->sync

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Date:   Fri, 10 Feb 2023 00:57:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        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 V2 4/8] kvm: x86/mmu: Set mmu->sync_page as NULL for
 direct paging

On Tue, Feb 07, 2023, Paolo Bonzini wrote:
> On 2/7/23 16:57, Lai Jiangshan wrote:
> > From: Lai Jiangshan<jiangshan.ljs@...group.com>
> > 
> > mmu->sync_page for direct paging is never called.
> > 
> > And both mmu->sync_page and mm->invlpg only make sense in shadowpaging.
> > Setting mmu->sync_page as NULL for direct paging makes it consistent
> > with mm->invlpg which is set NULL for the case.
> > 
> > Signed-off-by: Lai Jiangshan<jiangshan.ljs@...group.com>
> 
> I'd rather have a WARN_ON_ONCE in kvm_sync_page(), otherwise looks good.

Agreed, there's even a WARN_ON_ONCE() to piggyback:

	if (WARN_ON_ONCE(sp->role.direct || !vcpu->arch.mmu->sync_spte ||
			 (sp->role.word ^ root_role.word) & ~sync_role_ign.word))
		return -1;

Of course, the direct + match checks on the role effectively do the same, but I
would rather be explicit.