* Dave Hansen <dave.han...@linux.intel.com> wrote:
> > Btw., regarding XCR0.PKRU: that enables 'Protection Keys' in the
> > PTE format. What's the main purpose of these keys? They seem to
> > duplicate the read/write bits in the PTE, with the exception that
> > they don't impact instruction fetches. So is this used to allow
> > user-space to execute but otherwise not read instructions?
> >
> > Or some other purpose I missed?
>
> You can change the permissions of a given key with writes to the
> register without changing the PTE. No TLB shootdown, plus the
> permission changes are local to the CPU thread.
interesting ... where are we doing that? kmap_atomic() is the only
place I can think of in the kernel, but there we are already skipping
the TLB shootdown by doing an INVLPG.
> I have patches today if you're interested.
I'm always interested in new CPU features ;-)
> ...
> >> We'd need to determine which fields are in the init state before we
> >> do an xsaves.
> >
> > Why? I don't think that's necessary.
>
> "If RFBM[i] = 0, XSTATE_BV[i] is written as 0."
Yes, that's natural: RFBM is the combination of XCR0 (constant) and
the 'instruction mask' (constant as well) - i.e. it's a wide bitmask
including all our xfeatures - essentially 'xfeatures_mask' (in
tmp.fpu).
> We need to pull XSTATE_BV in to the instruction mask when doing an
> XSAVE* if our RFBM has bits unset that *are* set in XSTATE_BV.
> Otherwise, we'll destroy the bits at XSAVE* time.
But why would our RFBM be narrower than any possible XSTATE_BV we
handle? Our XCR0 is at the max.
Also, the XSTATE_BV value of the save area is immaterial when we do an
XSAVES: all the state is in CPU registers, we want to save it to the
save area. XSAVES will write it for us.
> It's not a problem today because the instruction mask is always -1,
> so it always has every bit set that *MIGHT* be set in XSTATE_BV.
Yes. And why would we ever want to narrow it?
> As for the whole bit 63 thing... It's possible and valid to have a
> XCOMP_BV[62:0]=0 because the instruction mask only contained bits
> that were unset in XCR0|IA32_XSS. You need bit 63 to tell you which
> format you are using.
So basically if you request an XSAVES to ... write nothing (XCR0 AND
instruction mask is 0), then it will represent this as 0|1<<63 in
XCOMP_BV?
In that case it does not matter whether the area is compacted or
standard: nothing was saved and nothing will have to be restored, only
the xsave header area will be accessed. Am I missing something?
Thanks,
Ingo
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