Re: [PATCH] KVM: x86: Ignore MSR_AMD64_BU_CFG access

Maciej S. Szmigiero
2023-10-05T14:46:30Z

On 5.10.2023 02:10, Sean Christopherson wrote:
> On Mon, Oct 02, 2023, Maciej S. Szmigiero wrote:
>> On 26.09.2023 00:25, Tom Lendacky wrote:
>>>>> It's partially documented in various AMD BKDGs, however I couldn't find
>>>>> any definition for this particular bit (8) - other than that it is reserved.
>>>>
>>>> I found it as MSR_AMD64_BU_CFG for Model 16h, but that's Jaguar/Puma, not Zen1.
>>>> My guess is that Windows is trying to write this thing:
>>>>
>>>>    MSRC001_1023 [Table Walker Configuration] (Core::X86::Msr::TW_CFG)
>>>>    Read-write. Reset: 0000_0000_0000_0000h.
>>>>    _lthree0_core[3,1]; MSRC001_1023
>>>>
>>>>    Bits   Description
>>>>    63:50  Reserved.
>>>>    49     TwCfgCombineCr0Cd: combine CR0_CD for both threads of a core. Read-write. Reset: 0. Init: BIOS,1.
>>>>           1=The host Cr0_Cd values from the two threads are OR'd together and used by both threads.
>>>>    48:0   Reserved.
>>>>
>>>> Though that still doesn't explain bit 8...  Perhaps a chicken-bit related to yet
>>>> another speculation bug?
>>>>
>>>> Boris or Tom, any idea what Windows is doing?  I doubt it changes our options in
>>>> terms of "fixing" this in KVM, but having a somewhat accurate/helpful changelog
>>>> would be nice.
>>>
>>> It's definitely not related to a speculation bug, but I'm unsure what was
>>> told to Microsoft that has them performing that WRMSR. The patch does the
>>> proper thing, though, as a guest shouldn't be updating that setting.
>>>
>>> And TW_CFG is the proper name of that MSR for Zen.
>>
>> So, should I prepare v2 with MSR_AMD64_BU_CFG -> MSR_AMD64_TW_CFG change?
>
> If we can get Paolo's attention, I'd like to get his thoughts on punting this
> to QEMU/userspace. I'm worried that "handling" uarch specific MSRs in KVM is
> going to paint us into a corner and force KVM to check guest F/M/S someday, which
> I want to avoid at pretty much all costs.

We already do similar ignoring in KVM for MSR_AMD64_BU_CFG2, MSR_AMD64_DC_CFG
and MSR_F15H_EX_CFG, so doing this {BU_CFG2,TW_CFG} MSR filtering in QEMU would
be inconsistent with these.

But let's wait for Paolo's opinion then.

Thanks,
Maciej