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

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Message-ID: <YKK3QLr2OsUdrX5D@google.com>
Date:   Mon, 17 May 2021 18:34:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jon Kohler <jon@...anix.com>, Babu Moger <babu.moger@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.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>,
        Linux Kernel Mailing List <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, Jim Mattson wrote:
> On Mon, May 17, 2021 at 11:05 AM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Mon, May 17, 2021, Dave Hansen wrote:
> > > On 5/17/21 10:49 AM, Andy Lutomirski wrote:
> > > >> 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.
> > > >
> > > > Or set a flag causing nmi_uaccess_ok() to return false.
> > >
> > > Oh, that doesn't sound too bad.  The VMENTER/EXIT paths are also
> > > essentially a context switch.
> >
> > I like that idea, too.
> >
> > The flag might also be useful to fix the issue where the NMI handler activates
> > PEBS after KVM disables it.  Jim?
> 
> The issue is actually that the NMI handler *clears* IA32_PEBS_ENABLE
> bits after giving out the host value of the MSR to KVM. If we were to
> block the NMI handler from modifying IA32_PEBS_ENABLE until after the
> next VM-exit, that could solve this issue. I don't know if it makes
> sense to piggyback on nmi_uaccess(), though.

I wasn't thinking about using nmi_uaccess_okay(), but rather whatever flag is
added so that can KVM can inform the NMI handler that KVM is in the middle of
its version of a context switch.

> > > Will widening the window where nmi_uaccess_okay()==false anger any of
> > > the perf folks?  It looks like perf knows how to handle it nicely.