Messages in this thread | | | Date | Wed, 16 Oct 2013 08:19:04 -0700 | Subject | Re: [PATCH, RFC] x86-64: properly handle FPU code/data selectors | From | Linus Torvalds <> |
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On Wed, Oct 16, 2013 at 5:00 AM, Jan Beulich <JBeulich@suse.com> wrote: > > The basic idea here is to either use a priori information on the > intended state layout (in the case of 32-bit processes) or "sense" the > proper layout (in the case of KVM guests) by inspecting the already > saved FPU rip/rdp, and reading their actual values in a second save > operation.
The rip/rdp thing looks very hacky. And *without* the rip/rdp thing, I think the word-size always matches the TIF32 bit, right?
Why wouldn't the high bits be zero even in 64-bit mode? It would seem to be a *major* bug if you are in 64-bit mode but (for example) try to use the low 32-bit of virtual memory (ie something x32-like), and now your patch decides to use the 32-bit layout.
As far as I can tell, you actually corrupt rid/rdp in that case (because when you write the fcs thing, it overwrites the high bits of rip, and fos overwrites the high bits of rdp). So now bits that *should* be zero are not.
So you're basically trying to save some old state by corrupting new state instead.
Am I overlooking something?
Linus
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