linux-kernel - Re: [PATCH v4 4/4] KVM: SVM: Add Bus Lock Detect support

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Message-ID: <5fe11dfb-2a33-4b52-8181-c82bc326b968@amd.com>
Date: Wed, 21 Aug 2024 16:10:47 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, pbonzini@...hat.com, thomas.lendacky@....com,
 jmattson@...gle.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 v4 4/4] KVM: SVM: Add Bus Lock Detect support

On 21-Aug-24 11:06 AM, Ravi Bangoria wrote:
>>> @@ -3158,6 +3159,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>>>  		if (data & DEBUGCTL_RESERVED_BITS)
>>
>> Not your code, but why does DEBUGCTL_RESERVED_BITS = ~0x3f!?!?  That means the
>> introduction of the below check, which is architecturally correct, has the
>> potential to break guests.  *sigh*
>>
>> I doubt it will cause a problem, but it's something to look out for.
> This dates back to 2008: https://git.kernel.org/torvalds/c/24e09cbf480a7
> 
> The legacy definition[1] of DEBUGCTL MSR is:
> 
>   5:2   PB: performance monitor pin control. Read-write. Reset: 0h.
>         This field does not control any hardware.
>   1     BTF. Read-write. Reset: 0. 1=Enable branch single step.
>   0     LBR. Read-write. Reset: 0. 1=Enable last branch record.
> 
> [1]: https://bugzilla.kernel.org/attachment.cgi?id=287389

How about adding cpu_feature_enabled() check:

	if (data & DEBUGCTL_RESERVED_BITS)
		return 1;

	if (cpu_feature_enabled(X86_FEATURE_BUS_LOCK_DETECT) &&
	    (data & DEBUGCTLMSR_BUS_LOCK_DETECT) &&
	    !guest_cpuid_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
		return 1;

Thanks,
Ravi