linux-kernel - Re: [PATCH] kvm: x86/svm/nested: Cache PDPTEs for nested NPT in PAE paging mode

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Date:   Thu, 26 May 2022 16:38:40 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Avi Kivity <avi@...hat.com>, kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH] kvm: x86/svm/nested: Cache PDPTEs for nested NPT in PAE
 paging mode

On Thu, May 26, 2022 at 5:45 AM David Matlack <dmatlack@...gle.com> wrote:
>
> On Mon, May 16, 2022 at 2:06 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Fri, Apr 15, 2022, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <jiangshan.ljs@...group.com>
> > >
> > > When NPT enabled L1 is PAE paging, vcpu->arch.mmu->get_pdptrs() which
> > > is nested_svm_get_tdp_pdptr() reads the guest NPT's PDPTE from memroy
> > > unconditionally for each call.
> > >
> > > The guest PAE root page is not write-protected.
> > >
> > > The mmu->get_pdptrs() in FNAME(walk_addr_generic) might get different
> > > values every time or it is different from the return value of
> > > mmu->get_pdptrs() in mmu_alloc_shadow_roots().
> > >
> > > And it will cause FNAME(fetch) installs the spte in a wrong sp
> > > or links a sp to a wrong parent since FNAME(gpte_changed) can't
> > > check these kind of changes.
> > >
> > > Cache the PDPTEs and the problem is resolved.  The guest is responsible
> > > to info the host if its PAE root page is updated which will cause
> > > nested vmexit and the host updates the cache when next nested run.
> >
> > Hmm, no, the guest is responsible for invalidating translations that can be
> > cached in the TLB, but the guest is not responsible for a full reload of PDPTEs.
> > Per the APM, the PDPTEs can be cached like regular PTEs:
> >
> >   Under SVM, however, when the processor is in guest mode with PAE enabled, the
> >   guest PDPT entries are not cached or validated at this point, but instead are
> >   loaded and checked on demand in the normal course of address translation, just
> >   like page directory and page table entries. Any reserved bit violations ared
> >   etected at the point of use, and result in a page-fault (#PF) exception rather
> >   than a general-protection (#GP) exception.
>
> This paragraph from the APM describes the behavior of CR3 loads while
> in SVM guest-mode. But this patch is changing how KVM emulates SVM
> host-mode (i.e. L1), right? It seems like AMD makes no guarantee
> whether or not CR3 loads pre-load PDPTEs while in SVM host-mode.
> (Although the APM does say that "modern processors" do not pre-load
> PDPTEs.)

Oh, I also missed the fact that L1 is the host when emulating it.

The code is for host-mode (L1)'s nested_cr3 which is using the
traditional PAE PDPTEs loading and checking.

So using caches is the only correct way, right?

If so, I can update this patch only (not adding it to the patchset of
one-off local shadow page) and add some checks to see if the loaded
caches changed.

Maybe I just ignore it since I'm not familiar with SVM enough.
I hope it served as a bug report.

Thanks
Lai