On 1/22/21 7:51 AM, Jan Beulich wrote:
> On 20.01.2021 23:49, Boris Ostrovsky wrote:
>
>> +
>> + /*
>> + * Accesses to unimplemented MSRs as part of emulation of instructions
>> + * other than guest's RDMSR/WRMSR should never succeed.
>> + */
>> + if ( !is_guest_msr_access )
>> + ignore_msrs = MSR_UNHANDLED_NEVER;
>
> Wouldn't you better "return true" here? Such accesses also
> shouldn't be logged imo (albeit I agree that's a change from
> current behavior).
Yes, that's why I didn't return here. We will be here in !is_guest_msr_access
case most likely due to a bug in the emulator so I think we do want to see the
error logged.
>
>> + if ( unlikely(ignore_msrs != MSR_UNHANDLED_NEVER) )
>> + *val = 0;
>
> I don't understand the conditional here, even more so with
> the respective changelog entry. In any event you don't
> want to clobber the value ahead of ...
>
>> + if ( likely(ignore_msrs != MSR_UNHANDLED_SILENT) )
>> + {
>> + if ( is_write )
>> + gdprintk(XENLOG_WARNING, "WRMSR 0x%08x val 0x%016"PRIx64
>> + " unimplemented\n", msr, *val);
>
> ... logging it.
True. I dropped !is_write from v1 without considering this.
As far as the conditional --- dropping it too would be a behavior change.
>
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.h
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.h
>> @@ -850,4 +850,10 @@ static inline void x86_emul_reset_event(struct
>> x86_emulate_ctxt *ctxt)
>> ctxt->event = (struct x86_event){};
>> }
>>
>> +static inline bool x86_emul_guest_msr_access(struct x86_emulate_ctxt *ctxt)
>
> The parameter wants to be pointer-to-const. In addition I wonder
> whether this wouldn't better be a sibling to
> x86_insn_is_cr_access() (without a "state" parameter, which
> would be unused and unavailable to the callers), which may end
> up finding further uses down the road.
"Sibling" in terms of name (yes, it would be) or something else?
>
>> +{
>> + return ctxt->opcode == X86EMUL_OPC(0x0f, 0x32) || /* RDMSR */
>> + ctxt->opcode == X86EMUL_OPC(0x0f, 0x30); /* WRMSR */
>> +}
>
> Personally I'd prefer if this was a single comparison:
>
> return (ctxt->opcode | 2) == X86EMUL_OPC(0x0f, 0x32);
>
> But maybe nowadays' compilers are capable of this
> transformation?
Here is what I've got (not an inline but shouldn't make much difference I'd
think)
ffff82d040385960 <x86_emul_guest_msr_access_2>: # your code
ffff82d040385960: 8b 47 2c mov 0x2c(%rdi),%eax
ffff82d040385963: 83 e0 fd and $0xfffffffd,%eax
ffff82d040385966: 3d 30 00 0f 00 cmp $0xf0030,%eax
ffff82d04038596b: 0f 94 c0 sete %al
ffff82d04038596e: c3 retq
ffff82d04038596f <x86_emul_guest_msr_access_1>: # my code
ffff82d04038596f: 8b 47 2c mov 0x2c(%rdi),%eax
ffff82d040385972: 83 c8 02 or $0x2,%eax
ffff82d040385975: 3d 32 00 0f 00 cmp $0xf0032,%eax
ffff82d04038597a: 0f 94 c0 sete %al
ffff82d04038597d: c3 retq
So it's a wash in terms of generated code.
>
> I notice you use this function only from PV priv-op emulation.
> What about the call paths through hvmemul_{read,write}_msr()?
> (It's also questionable whether the write paths need this -
> the only MSR written outside of WRMSR emulation is
> MSR_SHADOW_GS_BASE, which can't possibly reach the "unhandled"
> logic anywhere. But maybe better to be future proof here in
> case new MSR writes appear in the emulator, down the road.)
Won't we end up in hvm_funcs.msr_write_intercept ops which do call it?
-boris
