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From:   Jim Mattson <jmattson@google.com>
Date:   Mon, 17 May 2021 11:15:07 -0700
Message-ID: <CALMp9eS80a+Oy6spKT3cG7DCTW6jVwhyBuZ_t0SND=80Lg1XWA@mail.gmail.com>
Subject: Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave_state
To:     Sean Christopherson <seanjc@google.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jon Kohler <jon@nutanix.com>, Babu Moger <babu.moger@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Uros Bizjak <ubizjak@gmail.com>,
        Petteri Aimonen <jpa@git.mail.kapsi.fi>,
        Kan Liang <kan.liang@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Benjamin Thiel <b.thiel@posteo.de>,
        Fan Yang <Fan_Yang@sjtu.edu.cn>,
        Juergen Gross <jgross@suse.com>,
        Dave Jiang <dave.jiang@intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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On Mon, May 17, 2021 at 11:05 AM Sean Christopherson <seanjc@google.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.

> > 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.