Re: [Xen-devel] [PATCH] x86/xsave: simplify xcomp_bv initialization

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.12.15 at 11:53, <shuai.ruan@xxxxxxxxxxxxxxx> wrote:
> On Thu, Dec 17, 2015 at 09:58:38AM -0700, Jan Beulich wrote:
>> This eliminates a number of pointless conditionals: Bits 0 and 1 of
>> xcomp_bv don't matter anyway, and as long as none of bits 2..62 are
>> set, setting bit 63 is pointless too.
>> 
> We should set bit 63 of the xcomp_bv when initialization.
> From the initialization log of xen, I got that xrstors will 
> excute first(before xsaves). xrstors will check bit 63 of xcopm_bv. 
> If it is not set, on the xsaves-support machine xen will hung.
> (I test the patch on skylake machine base on the master branch)

Oh, indeed (assuming you mean crash instead of hang) - the
insn page says "#GP(0) ... If bit 63 of the XCOMP_BV field of the
XSAVE header is 0." I had only looked at the XRSTOR insn page.
Still I think we shouldn't drop the patch entirely, it just shouldn't
go as far as it does.

Jan


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