On 05/12/2018 16:39, Jan Beulich wrote:
>>>> On 03.12.18 at 17:18, <andrew.coop...@citrix.com> wrote:
>> At the time of writing, the spec is available from:
>>
>>
>> https://developer.amd.com/wp-content/resources/124441_AMD64_SpeculativeStoreB
>>
>> ypassDisable_Whitepaper_final.pdf
>>
>> Future hardware (Zen v2) is expect to have hardware MSR_SPEC_CTRL support,
>> including SPEC_CTRL.SSBD, and with the expectation that this will be directly
>> passed through to guests for performance.
>>
>> On currently released hardware, the only mechanism available is the legacy
>> LS_CFG option, and this is very expensive to use. Furthermore, emulating
>> MSR_SPEC_CTRL via interception is prohibitively expensive, as certain OSes
>> use
>> the write-discard flexibility to simplify their entry/exit logic.
> With this, ...
>
>> As an alternative, MSR_VIRT_SPEC_CTRL is specified as an architectural
>> control
>> (with semantics equivilent to MSR_SPEC_CTRL) which is provided by the
>> hypervisor. This abstracts away the model-specific details of the LS_CFG
>> mechanism, which allows migration safety to be retained.
> ... how is this any less expensive, when it necessarily requires
> interception?
For an individual update? No difference.
For the case where a certain OS takes the presence of MSR_SPEC_CTRL to
mean that it needs to write MSR_SPEC_CTRL.IBRS wherever it would choose
to if IBRS was actually available, the perf difference is in the number
of writes which occur and get intercepted.
> At least the way things are worded, I'm getting
> the impression that you consider this less expensive.
>
>> --- a/tools/libxl/libxl_cpuid.c
>> +++ b/tools/libxl/libxl_cpuid.c
>> @@ -245,6 +245,11 @@ int libxl_cpuid_parse_config(libxl_cpuid_policy_list
>> *cpuid, const char* str)
>> {"invtsc", 0x80000007, NA, CPUID_REG_EDX, 8, 1},
>>
>> {"ibpb", 0x80000008, NA, CPUID_REG_EBX, 12, 1},
>> + {"amd_ibrs", 0x80000008, NA, CPUID_REG_EBX, 14, 1},
> While of the following two the names indeed clash with Intel's, the
> above one doesn't. Any reason you still gave it an amd_ prefix?
I really really wish we didn't have duplicate bits, or that I'd started
with blanket Intel and AMD prefixes.
I deliberately chose "IBRSB" for the Intel bit due to its double
meaning, but using just IBRS here on its own is liable to get confused
with the Intel bit.
>
>> + {"amd_stibp", 0x80000008, NA, CPUID_REG_EBX, 15, 1},
>> + {"amd_ssbd", 0x80000008, NA, CPUID_REG_EBX, 24, 1},
>> + {"virt_sc_ssbd", 0x80000008, NA, CPUID_REG_EBX, 25, 1},
>> + {"amd_ssb_no", 0x80000008, NA, CPUID_REG_EBX, 26, 1},
> Since you're at it, why not also introduce names for bits 16-18
> at this occasion?
I haven't previously filled in names for the sake of it.
The reason that ibrs/stibp/ssbd are here is because they're related and
I've also got a followon few patches to support MSR_VIRT_SPEC_CTRL on
Rome hardware via MSR_SPEC_CTRL, but I need an SDP and some
experimentation time before I'd be happy posting them.
But to address your question, I can't locate those bits at all. Not
even in the NDA docs or Linux source.
~Andrew
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