linux-kernel - Re: [PATCH Part2 v5 37/45] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT

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Message-ID: <YWcWW7eikkWSmCkH@google.com>
Date:   Wed, 13 Oct 2021 17:24:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 37/45] KVM: SVM: Add support to handle MSR based
 Page State Change VMGEXIT

On Wed, Oct 13, 2021, Brijesh Singh wrote:
> > The more I look at this, the more strongly I feel that private <=> shared conversions
> > belong in the MMU, and that KVM's SPTEs should be the single source of truth for
> > shared vs. private.  E.g. add a SPTE_TDP_PRIVATE_MASK in the software available bits.
> > I believe the only hiccup is the snafu where not zapping _all_ SPTEs on memslot
> > deletion breaks QEMU+VFIO+GPU, i.e. KVM would lose its canonical info on unrelated
> > memslot deletion.
> >
> > But that is a solvable problem.  Ideally the bug, wherever it is, would be root
> > caused and fixed.  I believe Peter (and Marc?) is going to work on reproducing
> > the bug.
> We have been also setting up VM with Qemu + VFIO + GPU usecase to repro
> the bug on AMD HW and so far we no luck in reproducing it. Will continue
> stressing the system to recreate it. Lets hope that Peter (and Marc) can
> easily recreate on Intel HW so that we can work towards fixing it.

Are you trying on a modern kernel?  If so, double check that nx_huge_pages is off,
turning that on caused the bug to disappear.  It should be off for AMD systems,
but it's worth checking.

> >> +		if (!rc) {
> >> +			/*
> >> +			 * This may happen if another vCPU unmapped the page
> >> +			 * before we acquire the lock. Retry the PSC.
> >> +			 */
> >> +			write_unlock(&kvm->mmu_lock);
> >> +			return 0;
> > How will the caller (guest?) know to retry the PSC if KVM returns "success"?
> 
> If a guest is adhering to the GHCB spec then it will see that hypervisor
> has not processed all the entry and it should retry the PSC.

But AFAICT that information isn't passed to the guest.  Even in this single-page
MSR-based case, the caller will say "all good" on a return of 0.

The "full" path is more obvious, as the caller clearly continues to process
entries unless there's an actual failure.

+       for (; cur <= end; cur++) {
+               entry = &info->entries[cur];
+               gpa = gfn_to_gpa(entry->gfn);
+               level = RMP_TO_X86_PG_LEVEL(entry->pagesize);
+               op = entry->operation;
+
+               if (!IS_ALIGNED(gpa, page_level_size(level))) {
+                       rc = PSC_INVALID_ENTRY;
+                       goto out;
+               }
+
+               rc = __snp_handle_page_state_change(vcpu, op, gpa, level);
+               if (rc)
+                       goto out;
+       }