"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. > 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
