Re: [PATCH v2 3/4] x86: Allow non-faulting accesses to non-emulated MSRs if policy permits this

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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