Re: [Xen-devel] [PATCH v1 1/6] x86: Add support for STAC/CLAC instructions

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/04/14 09:07, Jan Beulich wrote:
>>>> On 22.04.14 at 09:41, <feng.wu@xxxxxxxxx> wrote:
>>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>>> That said, the macro contents itself is horrible too: A control register
>>> access and two conditional branches in code intended to be used in
>>> fast paths? Definitely not an option. Even the simplest possible
>>> solution - adding a global flag to be checked here - would already be
>>> questionable. Hence I think you should at least consider porting over
>>> proper instruction patching abstraction from Linux.
>>>
>> Jan, I did some investigation about how to handle this two instructions
>> in Linux, basically, it uses the alternatives mechanism to handle these
>> kind of cases. Let's take the following definition of ASM_STAC in Linux for 
>> example:
>>
>> #define ASM_CLAC                                                        \
>>         661: ASM_NOP3 ;                                                 \
>>         .pushsection .altinstr_replacement, "ax" ;                      \
>>         662: __ASM_CLAC ;                                               \
>>         .popsection ;                                                   \
>>         .pushsection .altinstructions, "a" ;                            \
>>         altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3 ;       \
>>         .popsection
>>
>> ASM_CLAC is defined as NOP by default, it puts the real CLAC instruction in 
>> section "altinstr_replacement" and
>> the needed information to " altinstructions " section, which is useful to 
>> replace the default
>> definition by the alternative one. Here is the routine call path: 
>> start_kernel () --> check_bugs() --> alternative_instructions().
>>
>> In function alternative_instructions(), it will check the related features 
>> in CPU, if it exists, the alternative definition will
>> overwrite the default one. So there is no conditional branches after this 
>> replacement when the Macro is being used.
>>
>> Do you think we need to port this whole mechanism to Xen to support 
>> CLAC/STAC? I am not sure if it is a little overkilled.
> Obviously we could use this machinery for other things. But whether it's
> needed here depends on the alternatives.
>
>> BTW, from the Linux implementation, I think we don't need to check the 'cr4' 
>> for the macros, we just need
>> to check whether the feature exists in the CPU. So is it acceptable to use 
>> the original code by eliminating the cr4 check?
> That _might_ be acceptable if you bring it down to just the three
> really necessary instructions: BT, JNC, CLAC/STAC. But the "might"
> has to stand - this, after all, remains an addition of a conditional
> branch (and for the performance of STAC/CLAC I haven't seen any
> documentation so far either) to several fast paths, and hence the
> patching alternative can't be discarded as the potentially better one.
>
> Jan

copy_{to,from}_guest() are already long paths (particularly for HVM) so
a single extra conditional is not going to be too bad (and as after boot
it will remain constant, the branch predictor will have a reliable time
with it).  It would certainly be fine for a v1 to get SMAP support working.

~Andrew

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