linux-kernel - Re: [PATCH 1/3] KVM: VMX: Retry APIC-access page reload if invalidation is in-progress

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Date:   Sat, 17 Jun 2023 11:45:00 +0800
From:   Yu Zhang <yu.c.zhang@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jason Gunthorpe <jgg@...dia.com>,
        Alistair Popple <apopple@...dia.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 1/3] KVM: VMX: Retry APIC-access page reload if
 invalidation is in-progress

> 
> > and the backing page is being reclaimed in L0? I saw
> > nested_get_vmcs12_pages() will check vmcs12 and set the APIC access address
> > in VMCS02, but not sure if this routine will be triggered by the mmu
> > notifier...
> 
> Pages from vmcs12 that are referenced by physical address in the VMCS are pinned
> (where "pinned" means KVM holds a reference to the page) by kvm_vcpu_map().  I.e.
> the page will not be migrated, and if userspace unmaps the page, userspace might
> break its VM, but that's true for any guest memory that userspace unexpectedly
> unmaps, and there won't be any no use-after-free issues.
> 
Thanks, Sean. 

About the kvm_vcpu_map(), is it necessary for APIC access address? L0 only
needs to get its pfn, and does not care about the hva or struct page. Could
we just use gfn_to_pfn() to retrieve the pfn, and kvm_release_pfn_clean() to
unpin it later? 

B.R.
Yu