linux-kernel - Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave

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Message-ID: <YKKqQZH7bX+7PDjX@google.com>
Date:   Mon, 17 May 2021 17:39:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Jon Kohler <jon@...anix.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Babu Moger <babu.moger@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Uros Bizjak <ubizjak@...il.com>,
        Petteri Aimonen <jpa@....mail.kapsi.fi>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Benjamin Thiel <b.thiel@...teo.de>,
        Fan Yang <Fan_Yang@...u.edu.cn>,
        Juergen Gross <jgross@...e.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under
 kvm_load_host_xsave_state

On Mon, May 17, 2021, Paolo Bonzini wrote:
> On 14/05/21 07:11, Andy Lutomirski wrote:
> > I don't even want to think about what happens if a perf NMI hits and
> > accesses host user memory while the guest PKRU is live (on VMX -- I
> > think this can't happen on SVM).
> 
> This is indeed a problem, which indeed cannot happen on SVM but is there on
> VMX.  Note that the function above is not handling all of the xstate, it's
> handling the *XSAVE state*, that is XCR0, XSS and PKRU.  Thus the window is
> small, but it's there.
> 
> Is it solvable at all, without having PKRU fields in the VMCS (and without
> masking NMIs in the LAPIC which would be too expensive)?  Dave, Sean, what
> do you think?

The least awful solution would be to have the NMI handler restore the host's
PKRU.  The NMI handler would need to save/restore the register, a la CR2, but the
whole thing could be optimized to run if and only if the NMI lands in the window
where the guest's PKRU is loaded.