Liu Yu wrote:
-----Original Message-----
From: Christian Ehrhardt [mailto:ehrhardt@xxxxxxxxxxxxxxxxxx]
Sent: Monday, August 11, 2008 4:33 PM
To: Liu Yu
Cc: Hollis Blanchard; kvm-ppc@xxxxxxxxxxxxxxx
Subject: Re: [PATCH 4 of 5] kvm: ppc: Write only modified
shadow entries into theTLB on exit
Liu Yu wrote:
-----Original Message-----
From: kvm-ppc-owner@xxxxxxxxxxxxxxx
[mailto:kvm-ppc-owner@xxxxxxxxxxxxxxx] On Behalf Of Hollis
Blanchard
Sent: Saturday, July 26, 2008 2:55 AM
To: avi@xxxxxxxxxxxx
Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
Subject: [PATCH 4 of 5] kvm: ppc: Write only modified
shadow entries
into theTLB on exit
@@ -229,6 +235,7 @@ void kvmppc_mmu_priv_switch(struct kvm_v
kvmppc_44x_shadow_release(vcpu, i);
stlbe->word0 = 0;
+ kvmppc_tlbe_set_modified(vcpu, i);
KVMTRACE_5D(STLB_INVAL, vcpu, i,
stlbe->tid, stlbe->word0, stlbe->word1,
stlbe->word2, handler);
Can this place use "_tlbia()" instead?
Hi Liu,
when we exit&reenter a guest on the lightweight path this
patch saves us from rebuilding too much of the tlb.
It's just the point of the patch to not invalidate all tlb
entries on every exit, but keeping as much as possible.
Additionally I'm not even sure if tlbia is available on all
cores used.
Am I overlooking something ? - what do you think would be the
benefit of using _tlbia() here?
Do you mean the loop to clear up to the hwater mark where
this patch makes changes and not this patch itself?
Yes.
When you set nearly all tlb entries modified in this loop,
you have to write all of them back before entering the guest in booke_interrupts.S.
So, why not "_tlbia()" here outside this loop, and don't set them modified.
Then, you needn't write them back.
In fact, the shadow tlb entries are still consistent with the hardware, as all of them are invalidated.
Am I overlooking something ?
As Hollis wrote in his patch comment usually just one tlb entry is
modified which saves a lot of updates.
If we invalidate all we later on would have to satisfy the misses we
cause by doing so.
With some free time we could test _tlbia() here, but I expect it to be
slower by causing the need for tlb re-population compared to what we
save here.
--
Grüsse / regards,
Christian Ehrhardt
IBM Linux Technology Center, Open Virtualization
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