linux-kernel - Re: FSGSBASE causing panic on 5.9-rc1

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Message-ID: <20200820220507.GA10269@sjchrist-ice>
Date:   Thu, 20 Aug 2020 15:05:07 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sasha Levin <sashal@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: FSGSBASE causing panic on 5.9-rc1

On Thu, Aug 20, 2020 at 01:36:46PM -0700, Andy Lutomirski wrote:
> 
> 
> > On Aug 20, 2020, at 1:15 PM, Tom Lendacky <thomas.lendacky@....com> wrote:
> > 
> > On 8/20/20 3:07 PM, Dave Hansen wrote:
> >> On 8/20/20 12:05 PM, Tom Lendacky wrote:
> >>>> I added a quick hack to save TSC_AUX to a new variable in the SVM
> >>>> struct and then restore it right after VMEXIT (just after where GS is
> >>>> restored in svm_vcpu_enter_exit()) and my guest is no longer crashing.
> >>> 
> >>> Sorry, I mean my host is no longer crashing.
> >> Just to make sure I've got this:
> >> 1. Older CPUs didn't have X86_FEATURE_RDPID
> >> 2. FSGSBASE patches started using RDPID in the NMI entry path when
> >>    supported *AND* FSGSBASE was enabled
> >> 3. There was a latent SVM bug which did not restore the RDPID data
> >>    before NMIs were reenabled after VMEXIT
> >> 4. If an NMI comes in the window between VMEXIT and the
> >>    wrmsr(TSC_AUX)... boom
> > 
> > Right, which means that the setting of TSC_AUX to the guest value needs to be moved, too.
> > 
> 
> Depending on how much of a perf hit this is, we could also skip using RDPID
> in the paranoid path on SVM-capable CPUs.

Doesn't this affect VMX as well?  KVM+VMX doesn't restore TSC_AUX until the
kernel returns to userspace.  I don't see anything that prevents the NMI
RDPID path from affecting Intel CPUs.

Assuming that's the case, I would strongly prefer this be handled in the
paranoid path.  NMIs are unblocked immediately on VMX VM-Exit, which means
using the MSR load lists in the VMCS, and I hate those with a vengeance.

Perf overhead on VMX would be 8-10% for VM-Exits that would normally stay
in KVM's run loop, e.g. ~125 cycles for the WMRSR, ~1300-1500 cycles to
handle the most common VM-Exits.  It'd be even higher overhead for the
VMX preemption timer, which is handled without even enabling IRQs and is
a hot path as it's used to emulate the TSC deadline timer for the guest.