linux-kernel - Re: [PATCH 3/3] KVM SVM: Add Bus Lock Detect support

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Message-ID: <735ced48-969e-4a62-8506-667a7491f4c9@amd.com>
Date: Thu, 11 Jul 2024 14:21:41 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Sean Christopherson <seanjc@...gle.com>, Jim Mattson <jmattson@...gle.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, pbonzini@...hat.com, thomas.lendacky@....com,
 hpa@...or.com, rmk+kernel@...linux.org.uk, peterz@...radead.org,
 james.morse@....com, lukas.bulwahn@...il.com, arjan@...ux.intel.com,
 j.granados@...sung.com, sibs@...natelecom.cn, nik.borisov@...e.com,
 michael.roth@....com, nikunj.dadhania@....com, babu.moger@....com,
 x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 santosh.shukla@....com, ananth.narayan@....com, sandipan.das@....com,
 manali.shukla@....com, Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH 3/3] KVM SVM: Add Bus Lock Detect support

On 10-Jul-24 7:22 PM, Sean Christopherson wrote:
> On Tue, Jul 09, 2024, Jim Mattson wrote:
>> On Tue, Jul 9, 2024 at 7:25 PM Ravi Bangoria <ravi.bangoria@....com> wrote:
>>>
>>> Sean,
>>>
>>> Apologies for the delay. I was waiting for Bus Lock Threshold patches to be
>>> posted upstream:
>>>
>>>   https://lore.kernel.org/r/20240709175145.9986-1-manali.shukla@amd.com
>>>
>>> On 12-Jun-24 7:12 AM, Sean Christopherson wrote:
>>>> On Wed, Jun 05, 2024, Ravi Bangoria wrote:
>>>>> On 6/5/2024 8:38 PM, Sean Christopherson wrote:
>>>>>> Some of the problems on Intel were due to the awful FMS-based feature detection,
>>>>>> but those weren't the only hiccups.  E.g. IIRC, we never sorted out what should
>>>>>> happen if both the host and guest want bus-lock #DBs.
>>>>>
>>>>> I've to check about vcpu->guest_debug part, but keeping that aside, host and
>>>>> guest can use Bus Lock Detect in parallel because, DEBUG_CTL MSR and DR6
>>>>> register are save/restored in VMCB, hardware cause a VMEXIT_EXCEPTION_1 for
>>>>> guest #DB(when intercepted) and hardware raises #DB on host when it's for the
>>>>> host.
>>>>
>>>> I'm talking about the case where the host wants to do something in response to
>>>> bus locks that occurred in the guest.  E.g. if the host is taking punitive action,
>>>> say by stalling the vCPU, then the guest kernel could bypass that behavior by
>>>> enabling bus lock detect itself.
>>>>
>>>> Maybe it's moot point in practice, since it sounds like Bus Lock Threshold will
>>>> be available at the same time.
>>>>
>>>> Ugh, and if we wanted to let the host handle guest-induced #DBs, we'd need code
>>>> to keep Bus Lock Detect enabled in the guest since it resides in DEBUG_CTL.  Bah.
>>>>
>>>> So I guess if the vcpu->guest_debug part is fairly straightforward, it probably
>>>> makes to virtualize Bus Lock Detect because the only reason not to virtualize it
>>>> would actually require more work/code in KVM.
>>>
>>> KVM forwards #DB to Qemu when vcpu->guest_debug is set and it's Qemu's
>>> responsibility to re-inject exception when Bus Lock Trap is enabled
>>> inside the guest. I realized that it is broken so I've prepared a
>>> Qemu patch, embedding it at the end.
>>>
>>>> I'd still love to see Bus Lock Threshold support sooner than later though :-)
>>>
>>> With Bus Lock Threshold enabled, I assume the changes introduced by this
>>> patch plus Qemu fix are sufficient to support Bus Lock Trap inside the
>>> guest?
>>
>> In any case, it seems that commit 76ea438b4afc ("KVM: X86: Expose bus
>> lock debug exception to guest") prematurely advertised the presence of
>> X86_FEATURE_BUS_LOCK to userspace on non-Intel platforms. We should
>> probably either accept these changes or fix up that commit. Either
>> way, something should be done for all active branches back to v5.15.
> 
> Drat.  Yeah, we need a patch to clear BUS_LOCK_DETECT in svm_set_cpu_caps(), marked
> for stable@.  Then this series can remove that clearing.
> 
> At least I caught it for CET[*]!  It'd be nice to not have to rely on humans to
> detect potential issues like this, but I can't think of a way to programmatically
> handle this situation without incurring an annoying amount of overhead and/or
> duplicate code between VMX and SVM.
> 
> [*] https://lore.kernel.org/all/ZjLRnisdUgeYgg8i@google.com

Sure, I'll add a patch and respin the series.

Thanks,
Ravi