On 07/09/2013 04:44:24 PM, Alexander Graf wrote:
On 09.07.2013, at 20:46, Scott Wood wrote:
> I suspect that tlbsx is faster, or at worst similar. And unlike
comparing tlbsx to lwepx (not counting a fix for the threading
problem), we don't already have code to search the guest TLB, so
testing would be more work.
We have code to walk the guest TLB for TLB misses. This really is
just the TLB miss search without host TLB injection.
So let's say we're using the shadow TLB. The guest always has its say
64 TLB entries that it can count on - we never evict anything by
accident, because we store all of the 64 entries in our guest TLB
cache. When the guest faults at an address, the first thing we do is
we check the cache whether we have that page already mapped.
However, with this method we now have 2 enumeration methods for guest
TLB searches. We have the tlbsx one which searches the host TLB and
we have our guest TLB cache. The guest TLB cache might still contain
an entry for an address that we already invalidated on the host.
Would that impose a problem?
I guess not because we're swizzling the exit code around to instead
be an instruction miss which means we restore the TLB entry into our
host's TLB so that when we resume, we land here and the tlbsx hits.
But it feels backwards.
Any better way? Searching the guest TLB won't work for the LRAT case,
so we'd need to have this logic around anyway. We shouldn't add a
second codepath unless it's a clear performance gain -- and again, I
suspect it would be the opposite, especially if the entry is not in
TLB0 or in one of the first few entries searched in TLB1. The tlbsx
miss case is not what we should optimize for.
At least this code has to become something more generic, such as
kvmppc_read_guest(vcpu, addr, TYPE_INSN) and move into the host mmu
implementation, as it's 100% host mmu specific.
I agree that e500_mmu_host.c is a better place for it (with an ifdef
for BOOKEHV), but supporting anything other than instruction fetches
could wait until we have a user for it (it means extra code to figure
out if permissions are correct).
-Scott
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
