Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

From: H. Peter Anvin
Date: Fri May 23 2008 - 02:02:34 EST

Suresh Siddha wrote:
On Thu, May 22, 2008 at 02:34:45PM -0700, H. Peter Anvin wrote:
Suresh Siddha wrote:
can you please elaborate? even in presence of virtualization, appropriate
cpuid bits need to be set/visible for application, for xsave/xrstor to work

For many paravirtualization solutions, CPUID "leak" from the hypervisor. The fact that CPUID cannot be disabled (made ring 0 only) is a major flaw in the architecture.

Therefore, relying on CPUID is too dangerous.

hmm.. so the kernel needs to export all the cpuid info (that the kernel enables
and supports) to the user through some mechanism then?

Not really. Most of the CPUID information doesn't have a OS-dependent portion, but the parts that do cause problems (there are other issues, like the hypervisor wanting to limit the feature set available to the client, but there is no real solution to that, and certainly it would be out of scope for this discussion.)

atleast in the xsave case, hypervisor can completely control the
OSXSAVE flag. I am still not convinced whether I need to add prctl()
to indicate the layout. If I have to add, then it should not just
be about whether xsave information is included in _fpstate or not, it should also
be about the whole cpuid information provided by the xsave architecture. Because
the user potentially needs all that information, to make sense out of
the data layout included in the extended state area.

Agreed that this information is needed, although it probably would be better to leave it in the frame itself; that way at least the rt frames would stand on their own. I'm worried about using the reserved fields since they could be messed up by ptrace, but you could also argue that if you have the ability to trace the process you can do anything else to it; that way we could use a reserved field to signal the presence of the XSAVE information in the frame; I would hope that that XSAVE information would be self-describing (perhaps a linked list of blocks or a separate chain of descriptors.)

In other words, we know of a place to stash the presence flag for rt signal frames, the uc_flags field; we effectively need to find a place to stash the uc_flags field for the non-rt frame.

Michael, you wrote:

Hmm, right now it seems this field has a de-facto ABI of being
either 0xffff (plain) or 0x0000 (fxsr). Using other values would
confuse at least one application I know of. Sad.

... could you share what application that is? This otherwise would be ideal.

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