Re: [PATCH v2] KVM: x86/pmu: Fix emulation on Intel counters' bit width

[Like Xu <like.xu.linux@gmail.com>]

On 27/3/2023 10:30 pm, Paolo Bonzini wrote:
> On Wed, Mar 22, 2023 at 10:31 AM Like Xu <like.xu.linux@gmail.com> wrote:
>>
>> From: Like Xu <likexu@tencent.com>
>>
>> Per Intel SDM, the bit width of a PMU counter is specified via CPUID
>> only if the vCPU has FW_WRITE[bit 13] on IA32_PERF_CAPABILITIES.
>> When the FW_WRITE bit is not set, only EAX is valid and out-of-bounds
>> bits accesses do not generate #GP. Conversely when this bit is set, #GP
>> for out-of-bounds bits accesses will also appear on the fixed counters.
>> vPMU currently does not support emulation of bit widths lower than 32
>> bits or higher than its host capability.
> 
> Can you please point out the date and paragraph of the SDM?
> 
> Paolo
> 

25462-078US, December 2022
20.2.6 Full-Width Writes to Performance Counter Registers

The general-purpose performance counter registers IA32_PMCx are writable via 
WRMSR instruction.
However, the value written into IA32_PMCx by WRMSR is the signed extended 64-bit 
value of the
EAX[31:0] input of WRMSR.

A processor that supports full-width writes to the general-purpose performance 
counters enumerated by
CPUID.0AH:EAX[15:8] will set IA32_PERF_CAPABILITIES[13] to enumerate its 
full-width-write
capability See Figure 20-65.

If IA32_PERF_CAPABILITIES.FW_WRITE[bit 13] =1, each IA32_PMCi is accompanied by a
corresponding alias address starting at 4C1H for IA32_A_PMC0.

The bit width of the performance monitoring counters is specified in 
CPUID.0AH:EAX[23:16].
If IA32_A_PMCi is present, the 64-bit input value (EDX:EAX) of WRMSR to 
IA32_A_PMCi will cause
IA32_PMCi to be updated by:

	COUNTERWIDTH =
		CPUID.0AH:EAX[23:16] bit width of the performance monitoring counter 		
	IA32_PMCi[COUNTERWIDTH-1:32] := EDX[COUNTERWIDTH-33:0]);
	IA32_PMCi[31:0] := EAX[31:0];
	EDX[63:COUNTERWIDTH] are reserved

---

Some might argue that this is all talking about GP counters, not fixed counters.
In fact, the full-width write hw behaviour is presumed to do the same thing for 
all counters.

Commercial hardware will not use less than 32 bits or a bit width like 46 bits.
A KVM user space (such as selftests) may set a strange bit-width, for example 
using 33 bits,
and based on the current code, writing the reserved bits for #fixed counters 
doesn't cause #GP.

Also when the guest does not have the Full-Width feature, the fixed counters can 
be more than
32 bits wide via CPUID, while the #GP counter is only 32 bits wide, which is 
also monstrous.

The current KVM is also not capable of emulating counter overflow when KVM user 
space is set
to a bit width of less than 32 bits w/ FW_WRITE.

The above SDM-undefined behaviour led to this fix, which may lift some of the fog.