linux-kernel - Re: [PATCH 09/12] KVM: MMU: look for a cached PGD when going from 32-bit to 64-bit

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Message-ID: <Ygae/V/UxD6axX2G@google.com>
Date:   Fri, 11 Feb 2022 17:38:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, mlevitsk@...hat.com, dmatlack@...gle.com
Subject: Re: [PATCH 09/12] KVM: MMU: look for a cached PGD when going from
 32-bit to 64-bit

On Fri, Feb 11, 2022, Paolo Bonzini wrote:
> On 2/11/22 02:32, Sean Christopherson wrote:
> > Maybe cached_root_find_and_rotate() or cached_root_find_and_age()?
> 
> I'll go for cached_root_find_and_keep_current() and
> cached_root_find_without_current(), respectively.
> 
> > 
> > Hmm, while we're refactoring this, I'd really prefer we not grab vcpu->arch.mmu
> > way down in the helpers.  @vcpu is needed only for the request, so what about
> > doing this?
> > 
> > 	if (!fast_pgd_switch(vcpu, new_pgd, new_role)) {
> > 		/*
> > 		 * <whatever kvm_mmu_reload() becomes> will set up a new root
> > 		 * prior to the next VM-Enter.  Free the current root if it's
> > 		 * valid, i.e. if a valid root was evicted from the cache.
> > 		 */
> > 		if (VALID_PAGE(vcpu->arch.mmu->root.hpa))
> > 			kvm_mmu_free_roots(vcpu, vcpu->arch.mmu, KVM_MMU_ROOT_CURRENT);
> > 		return;
> > 	}
> 
> I tried, but it's much easier to describe the cache functions if their
> common postcondition is "vcpu->arch.mmu->root.hpa is never stale"; which
> requires not a struct kvm_vcpu* but at least a struct kvm*, for the MMU
> lock.
> 
> I could change kvm_mmu_free_roots and cached_root_* to take a struct kvm*
> plus a struct kvm_mmu*.  Does that sound better?

Ya, works for me, thanks!