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

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Date:   Tue, 17 May 2022 09:10:50 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     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>,
        "open list:KERNEL VIRTUAL MACHINE FOR MIPS (KVM/mips)" 
        <kvm@...r.kernel.org>
Subject: Re: [PATCH] kvm: x86/svm/nested: Cache PDPTEs for nested NPT in PAE
 paging mode

On Tue, May 17, 2022 at 9:02 AM Lai Jiangshan <jiangshanlai@...il.com> wrote:
>
> On Tue, May 17, 2022 at 4:45 AM 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.
> >
> > So if L1 modifies a PDPTE from !PRESENT (or RESERVED) to PRESENT (and valid), then
> > any active L2 vCPUs should recognize the new PDPTE without a nested VM-Exit because
> > the old entry can't have been cached in the TLB.
>
> In this case, it is still !PRESENT in the shadow page, and it will cause
> a vmexit when L2 tries to use the translation.  I can't see anything wrong
> in TLB or vTLB(shadow pages).
>
> But I think some code is needed to reload the cached PDPTEs
> when guest_mmu->get_pdptrs() returns !PRESENT and reload mmu if
> the cache is changed. (and add code to avoid infinite loop)
>
> The patch fails to reload mmu if the cache is changed which
> leaves the problem described in the changelog partial resolved
> only.
>
> Maybe we need to add mmu parameter back in load_pdptrs() for it.
>

It is still too complicated, I will try to do a direct check
in FNAME(fetch) instead of (re-)using the cache.

> >
> > In practice, snapshotting at nested VMRUN would likely work, but architecturally
> > it's wrong and could cause problems if L1+L2 are engange in paravirt shenanigans,
> > e.g. async #PF comes to mind.
> >
> > I believe the correct way to fix this is to write-protect nNPT PDPTEs like all other
> > shadow pages, which shouldn't be too awful to do as part of your series to route
> > PDPTEs through kvm_mmu_get_page().
>
> In the one-off special shadow page (will be renamed to one-off local
> shadow page)
> patchsets, PAE PDPTEs is not write-protected.  Wirte-protecting it causing nasty
> code.