On 11/22/2011 03:45 PM, Yoder Stuart-B08248 wrote:
> Hmm...not sure which approach makes sense.
>
> So to make sure I understand Scott's proposal the sequence would
> be something like:
>
> -after secondaries CPUs are released from spin and can
> determine they are running under KVM they:
> -disable interrupts
> -set a flag indicating they are waiting for paravirt
> patching to complete
> -spin in a loop until a flag is set that indicates
> paravirt patch is complete
> -make the paravirt hcall
>
> -boot CPU
> -waits for all secondaries to set the 'waiting for
> paravirt' flag
> -makes the paravirt hcall
> -patches the kernel
> -sets 'paravirt complete' flag
>
> Is that basically correct?
That's not my proposal.
My proposal is just to add something like this at the end of
kvm_map_magic_page():
if (not the cpu doing the patching) {
while (!kvm_patching_done)
cpu_relax();
}
And even that is only really worth it if there are races besides SPRG3-7
to worry about. Using the privileged version of SPRG3-7 inside the
kernel would avoid needing anything special for KVM, and would make SPRG
access slightly simpler than they are now (no separate W/R versions to
keep track of) -- I'm not sure why the kernel bothers with the user-read
version of the SPRs in the first place.
-Scott
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