Re: [Xen-devel] [PATCHv2 1/3] x86/fpu: improve check for XSAVE* not writing FIP/FDP fields

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.02.16 at 11:37, <kevin.tian@xxxxxxxxx> wrote:
> Sorry I didn't quite get the question here. Could anyone of you
> write down a standalone description of the problem then I can
> forward internally to confirm since my translation might be
> inaccurate here?

What we'd like to get formally stated is whether FIP is guaranteed
to be treated as 48-bit pointer, which upon loading/storing by
64-bit {F,}X{XSAVE,RSTOR} will get truncated/canonicalized. With
FDP being a full 64-bit pointer on Intel CPUs (but only a 48 bit one
on AMD ones), and both your and their manuals implicitly describing
both as full 64-bit fields, FIP potentially also being a full 64-bit field
on past, present, or future CPUs would render David's intended
code improvement unsafe.

Jan


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