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

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Message-ID: <YKKwSLnkzc77HcnG@google.com>
Date:   Mon, 17 May 2021 18:04:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     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>,
        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>,
        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, 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.