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    SubjectRe: [RFC] x86: xsave/xrstor support, ucontext_t extensions
    Suresh Siddha wrote:
    >
    > hpa, What is the virtualization problem? Are you referring to perf problem?
    > As you noted, regular non-rt signal handlers won't need this cpuid check. It's
    > needed only for those who manually look at non-rt signal frames and interpret it.
    > And also, they can do this check only once and not everytime.
    >

    No, relying on CPUID and vdso both have implications for virtualization.

    > To me, prtcl() just seems to be an overkill.

    I don't think it is ... it's not overkill but rather "underkill"... it's
    a low-performance solution but it's guaranteed to be safe in the
    presence of virtualization of all its various ilk. Note that you don't
    need to be able to *set* the format via prctl(), just *query* (get) it.

    Using prctl() allows us to make this personality-dependent if we ever
    need to.

    > While restoring from the user, kernel also need to find out what layout
    > the user is passing. So it's bi-directional. I prefer the same mechanism
    > (using cookies/magic numbers etc inaddition to uc_flags or cpuid checks) to
    > interpret the fpstate for both user/kernel.

    No, it really doesn't: the kernel only needs to be able to read the same
    format as it itself wrote.

    > ARM also seem to be using similar things while extending their ucontext_t,
    > with out other kernel interfaces to indicate the layout.
    >
    > BTW, how come 32bit kernel doesn't have the X86_FXSR_MAGIC checks, while restoring
    > the extended fxsave data from _fpstate?

    Again, the kernel already knows the format, so it doesn't need to check.

    -hpa


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