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Date:   Mon, 17 May 2021 18:04:56 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     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>,
        Jim Mattson <jmattson@google.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>,
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        Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>
Subject: Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under
 kvm_load_host_xsave_state
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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?

> 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.