Messages in this thread | | | Date | Thu, 5 Oct 2023 12:50:24 +0200 | Subject | Re: [PATCH] KVM: x86: Ignore MSR_AMD64_BU_CFG access | From | "Maciej S. Szmigiero" <> |
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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
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