Re: [PATCH] i386: Disable BTS and PEBS

[Paolo Bonzini <pbonzini@redhat.com>]

On 7/18/22 22:12, Sean Christopherson wrote:
> On Mon, Jul 18, 2022, Paolo Bonzini wrote:
>> This needs to be fixed in the kernel because old QEMU/new KVM is supported.
> 
> I can't object to adding a quirk for this since KVM is breaking userspace, but on
> the KVM side we really need to stop "sanitizing" userspace inputs unless it puts
> the host at risk, because inevitably it leads to needing a quirk.

The problem is not the sanitizing, it's that userspace literally cannot 
know that this needs to be done because the feature bits are "backwards" 
(1 = unavailable).

The right way to fix it is probably to use feature MSRs and, by default, 
leave the features marked as unavailable.  I'll think it through and 
post a patch tomorrow for both KVM and QEMU (to enable PEBS).

>> But apart from that, where does Linux check MSR_IA32_MISC_ENABLE_BTS_UNAVAIL
>> and MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL?
> 
> The kernel uses synthetic feature flags that are set by:
> 
>    static void init_intel(struct cpuinfo_x86 *c)
> 
> 	if (boot_cpu_has(X86_FEATURE_DS)) {
> 		unsigned int l1, l2;
> 
> 		rdmsr(MSR_IA32_MISC_ENABLE, l1, l2);
> 		if (!(l1 & (1<<11)))
> 			set_cpu_cap(c, X86_FEATURE_BTS);
> 		if (!(l1 & (1<<12)))
> 			set_cpu_cap(c, X86_FEATURE_PEBS);
> 	}

Gah, shift constants are evil.   I sent 
https://lore.kernel.org/all/20220719174714.2410374-1-pbonzini@redhat.com/ to 
clean this up.

Paolo

> and consumed by:
> 
>    void __init intel_ds_init(void)
> 
> 	/*
> 	 * No support for 32bit formats
> 	 */
> 	if (!boot_cpu_has(X86_FEATURE_DTES64))
> 		return;
> 
> 	x86_pmu.bts  = boot_cpu_has(X86_FEATURE_BTS);
> 	x86_pmu.pebs = boot_cpu_has(X86_FEATURE_PEBS);
> 	x86_pmu.pebs_buffer_size = PEBS_BUFFER_SIZE;
>