lkml.org 
[lkml]   [2022]   [Oct]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for init_fpstate
From
On 10/13/2022 10:21 AM, Dave Hansen wrote:
> On 10/13/22 09:23, Chang S. Bae wrote:
>>
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -1127,8 +1127,12 @@ void __copy_xstate_to_uabi_buf(struct membuf to,
>> struct fpstate *fpstate,
>>          * non-compacted format disabled features still occupy state space,
>>          * but there is no state to copy from in the compacted
>>          * init_fpstate. The gap tracking will zero these states.
>> +        *
>> +        * In the case of guest fpstate, this user_xfeatures does not
>> +        * dynamically reflect the capacity of the XSAVE buffer but
>> +        * xfeatures does. So AND them together.
>>          */
>> -       mask = fpstate->user_xfeatures;
>> +       mask = fpstate->user_xfeatures & fpstate->xfeatures;
>
> I'm not sure this is quite right either.
>
> Doesn't kvm expect that all of the ->user_xfeatures will end up being
> copied out? We surely can't copy them from 'fpstate' if the feature
> isn't there, but we can't skip them entirely, can we?

No, we can't skip them. IIUC, the code will zero out:

/*
* ... The gap tracking will zero these states.
*/
mask = fpstate->user_xfeatures;

for_each_extended_xfeature(i, mask) {
/*
* If there was a feature or alignment gap, zero the space
* in the destination buffer.
*/
if (zerofrom < xstate_offsets[i])
membuf_zero(&to, xstate_offsets[i] - zerofrom);

<snip>

/*
* Keep track of the last copied state in the non-compacted
* target buffer for gap zeroing.
*/
zerofrom = xstate_offsets[i] + xstate_sizes[i];
}

out:
if (to.left)
membuf_zero(&to, to.left);

Thanks,
Chang

\
 
 \ /
  Last update: 2022-10-13 19:34    [W:0.100 / U:0.704 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site