Re: [PATCH] i386: Disable BTS and PEBS

[Sean Christopherson <seanjc@google.com>]

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.

> 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);
	}

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;