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Re: [Xen-devel] [PATCH 06/10] x86/spec_ctrl: Split X86_FEATURE_SC_MSR into PV and HVM variants
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 16 May 2018 11:28:14 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 16 May 2018 10:28:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 16/05/18 07:38, Jan Beulich wrote:
>>>> On 15.05.18 at 21:52, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 14/05/18 16:27, Jan Beulich wrote:
>>>>>> On 11.05.18 at 12:38, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> --- a/xen/arch/x86/spec_ctrl.c
>>>> +++ b/xen/arch/x86/spec_ctrl.c
>>>> @@ -128,7 +128,8 @@ static void __init print_details(enum ind_thunk thunk,
>>>> uint64_t caps)
>>>> thunk == THUNK_RETPOLINE ? "RETPOLINE" :
>>>> thunk == THUNK_LFENCE ? "LFENCE" :
>>>> thunk == THUNK_JMP ? "JMP" : "?",
>>>> - boot_cpu_has(X86_FEATURE_SC_MSR) ?
>>>> + (boot_cpu_has(X86_FEATURE_SC_MSR_PV) ||
>>>> + boot_cpu_has(X86_FEATURE_SC_MSR_HVM)) ?
>>>> default_xen_spec_ctrl & SPEC_CTRL_IBRS ? " IBRS+" :
>>>> " IBRS-" : "",
>>>> opt_ibpb ? " IBPB" : "",
>>>> @@ -367,7 +368,8 @@ void __init init_speculation_mitigations(void)
>>>> * need the IBRS entry/exit logic to virtualise IBRS support for
>>>> * guests.
>>>> */
>>>> - setup_force_cpu_cap(X86_FEATURE_SC_MSR);
>>>> + setup_force_cpu_cap(X86_FEATURE_SC_MSR_PV);
>>>> + setup_force_cpu_cap(X86_FEATURE_SC_MSR_HVM);
>>> Besides these sort of open coding alternative_io_2() (you'd really want an
>>> output-less variant here, I agree) these are slightly bending the rules of
>>> when/how to use multiple alternatives: The above ends up correct only
>>> because of both replacements being identical.
>> Actually, by reordering patch 10 ahead of this patch, we never get to
>> needing the ALTERNATIVE_2()'s in the first place, and lose any concerns
>> with bending the rules along the series.
> Ah yes, indeed. And you would better use alternative_input() there then,
> instead of open coding it.
The reason this doesn't use alternative_input() at the moment is because
of the memory clobber. (And the lack of a memory clobber is called out
as a peculiarity in comment). The current code looks dangerously
inconsistent WRT barriers.
As for bending the rules, I now disagree with your assessment. The
alternative_*() wrappers do nothing but make it harder to express the
parameters, as perfectly demonstrated by the ASM_OUTPUT2() bodge.
I don't see their value, and they have a cost of making an asm volatile
statement not look and work quite as an asm volatile statement does in
all other callsites.
~Andrew
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