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

From: Maciej S. Szmigiero
Date: Thu Oct 05 2023 - 10:46:37 EST


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