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

[Suresh Siddha]

On Thu, May 22, 2008 at 02:02:28PM -0700, H. Peter Anvin wrote:
> 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.

can you please elaborate? even in presence of virtualization, appropriate
cpuid bits need to be set/visible for application, for xsave/xrstor to work
properly.

> 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.

But user can potentially change the _fpstate pointer in sigcontext struct.

> 
> >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.

What happens with some old applications which change the _fpstate
pointer. they may or may not be fxsave aware...

thanks,
suresh
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