Re: [Xen-devel] [V3 PATCH 7/9] x86/hvm: pkeys, add pkeys support for guest_walk_tables

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.12.15 at 10:03, <huaitong.han@xxxxxxxxx> wrote:
> On Wed, 2015-12-16 at 01:32 -0700, Jan Beulich wrote:
>> > > > On 16.12.15 at 09:16, <huaitong.han@xxxxxxxxx> wrote:
>> > On Tue, 2015-12-15 at 02:02 -0700, Jan Beulich wrote:
>> > > Well, I wouldn't want you to introduce a brand new function, but
>> > > instead just factor out the necessary piece from xsave() (making
>> > > the new one take a struct xsave_struct * instead of a struct vcpu
>> > > *,
>> > > and calling it from what is now xsave()).
>> > So the function looks like this:
>> > unsigned int get_xsave_pkru(struct vcpu *v)
>> > {
>> >     void *offset;
>> >     struct xsave_struct *xsave_area;
>> >     uint64_t mask = XSTATE_PKRU;
>> >     unsigned int index = fls64(mask) - 1;
>> >     unsigned int pkru = 0;
>> > 
>> >     if ( !cpu_has_xsave )
>> >         return 0;
>> >     
>> >     BUG_ON(xsave_cntxt_size < XSTATE_AREA_MIN_SIZE);
>> >     xsave_area = _xzalloc(xsave_cntxt_size, 64);
>> >     if ( xsave_area == NULL )
>> >         return 0;
>> > 
>> >     xsave(xsave_area, mask);
>> >     offset = (void *)xsave_area + (xsave_area_compressed(xsave) ? 
>> >             XSTATE_AREA_MIN_SIZE : xstate_offsets[index] );
>> >     memcpy(&pkru, offset, sizeof(pkru));
>> > 
>> >     xfree(xsave_area);
>> > 
>> >     return pkru;
>> > }
>> 
>> Depending on how frequently this might get called, the allocation
>> overhead may not be tolerable. I.e. you may want to set up e.g.
>> a per-CPU buffer up front. Or you check whether using RDPKRU
>> (with temporarily setting CR4.PKE) is cheaper than what you
>> do right now.
> RDPKRU does cost less than the function, and if temporarily setting
> CR4.PKE is accepted, I will use RDPKRU instead of the function.

The question isn't just the RDPKRU cost, but that of the two CR4
writes.

Jan


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