Sean Christopherson writes: > "KVM: x86:" for the shortlog scope. > > On Sat, May 21, 2022, Lev Kujawski wrote: >> Certain guest operating systems (e.g., UNIXWARE) clear bit 0 of >> MC1_CTL to ignore single-bit ECC data errors. > > Not that it really matters, but is this behavior documented anywhere? I've searched > a variety of SDMs, APMs, and PPRs, and can't find anything that documents this exact > behavior. I totally believe that some CPUs behave this way, but it'd be nice to > document exactly which generations of whose CPUs allow clearing bit zero. Intel's coverage of IA32_MC1_CTL appears to be proprietary (perhaps Appendix H material), but AMD helpfully documented it on page 204 of their BIOS and Kernel Developer's Guide: https://www.amd.com/system/files/TechDocs/26094.PDF I experimentally determined that UNIXWARE writes MC1_CTL on QEMU models "pentium2" or newer, but my guess is that this functionality was actually introduced with the Pentium Pro. >> Single-bit ECC data errors are always correctable and thus are safe to ignore >> because they are informational in nature rather than signaling a loss of data >> integrity. >> >> Prior to this patch, these guests would crash upon writing MC1_CTL, >> with resultant error messages like the following: >> >> error: kvm run failed Operation not permitted >> EAX=fffffffe EBX=fffffffe ECX=00000404 EDX=ffffffff >> ESI=ffffffff EDI=00000001 EBP=fffdaba4 ESP=fffdab20 >> EIP=c01333a5 EFL=00000246 [---Z-P-] CPL=0 II=0 A20=1 SMM=0 HLT=0 >> ES =0108 00000000 ffffffff 00c09300 DPL=0 DS [-WA] >> CS =0100 00000000 ffffffff 00c09b00 DPL=0 CS32 [-RA] >> SS =0108 00000000 ffffffff 00c09300 DPL=0 DS [-WA] >> DS =0108 00000000 ffffffff 00c09300 DPL=0 DS [-WA] >> FS =0000 00000000 ffffffff 00c00000 >> GS =0000 00000000 ffffffff 00c00000 >> LDT=0118 c1026390 00000047 00008200 DPL=0 LDT >> TR =0110 ffff5af0 00000067 00008b00 DPL=0 TSS32-busy >> GDT= ffff5020 000002cf >> IDT= ffff52f0 000007ff >> CR0=8001003b CR2=00000000 CR3=0100a000 CR4=00000230 >> DR0=00000000 DR1=00000000 DR2=00000000 DR3=00000000 >> DR6=ffff0ff0 DR7=00000400 >> EFER=0000000000000000 >> Code=08 89 01 89 51 04 c3 8b 4c 24 08 8b 01 8b 51 04 8b 4c 24 04 <0f> >> 30 c3 f7 05 a4 6d ff ff 10 00 00 00 74 03 0f 31 c3 33 c0 33 d2 c3 8d >> 74 26 00 0f 31 c3 >> >> Signed-off-by: Lev Kujawski <lkujaw@xxxxxxxxxxxxxx> >> --- >> arch/x86/kvm/x86.c | 7 +++++-- >> 1 file changed, 5 insertions(+), 2 deletions(-) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index 4790f0d7d40b..128dca4e7bb7 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -3215,10 +3215,13 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info) >> /* only 0 or all 1s can be written to IA32_MCi_CTL >> * some Linux kernels though clear bit 10 in bank 4 to >> * workaround a BIOS/GART TBL issue on AMD K8s, ignore >> - * this to avoid an uncatched #GP in the guest >> + * this to avoid an uncatched #GP in the guest. >> + * >> + * UNIXWARE clears bit 0 of MC1_CTL to ignore >> + * correctable, single-bit ECC data errors. >> */ >> if ((offset & 0x3) == 0 && >> - data != 0 && (data | (1 << 10)) != ~(u64)0) >> + data != 0 && (data | (1 << 10) | 1) != ~(u64)0) >> return -1; > > If KVM injects a #GP like it's supposed to[*], will UNIXWARE eat the #GP and continue > on, or will it explode? If it continues on, I'd prefer to avoid more special casing in > KVM. > > If it explodes, I think my preference would be to just drop the MCi_CTL checks > entirely. AFAICT, P4-based and P5-based Intel CPus, and all? AMD CPUs allow > setting/clearing arbitrary bits. The checks really aren't buying us anything, > and it seems like Intel retroactively defined the "architectural" behavior of > only 0s/1s. > > [*] https://lore.kernel.org/all/20220512222716.4112548-2-seanjc@xxxxxxxxxx Unfortunately, I cannot say if the UNIXWARE kernel would panic because QEMU enters a STOP state from which attempts to continue are met with "Error: Resetting the Virtual Machine is required." Thanks for the feedback, Lev
