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SubjectRe: [kvm-devel] [BUG] Oops with KVM-27
Il Sat, Jun 16, 2007 at 10:43:23AM +0300, Avi Kivity ha scritto: 
> Luca Tettamanti wrote:
> > Il Fri, Jun 15, 2007 at 12:06:50PM +0300, Avi Kivity ha scritto:
> >
> >>> After a bit of thinking: it's correct but removes an optimization;
> >>> furthermore it may miss other instructions that write to memory mapped
> >>> areas.
> >>> A more proper fix should be "force the writeback if dst.ptr is in some
> >>> kind of mmio area".
> >>>
> >>>
> >> I think we can just disable the optimization, and (in a separate patch)
> >> add it back in emulator_write_phys(), where we know we're modifying
> >> memory and not hitting mmio.
> >>
> >
> > Problem is that in emulator_write_phys() we've already lost track of the
> > previous (dst.orig_val) value. I moved up the decision in
> >
>
> Actually we haven't; just before the memcpy(), we can put a memcmp() to
> guard the kvm_mmu_pte_write(), which is the really expensive operation,
> especially with guest smp.

Yup, but it seemed wasteful to map (at least when highmem is in use) a
page just to check for something that we already knew. That was a
preemptive optmization though, I haven't actually benchmarked the cost
of setting up the mapping ;-)

[...]
> I think we can simply remove the if (). For the register case, the
> check is more expensive that the write; for mmio, we don't want it; and
> for memory writes, we can put it in emulator_write_phys().

Ok, this way it's simpler. How does this look:

--- a/kernel/x86_emulate.c 2007-06-15 21:13:51.000000000 +0200
+++ b/kernel/x86_emulate.c 2007-06-17 16:57:50.000000000 +0200
@@ -1057,40 +1057,38 @@
}

writeback:
- if ((d & Mov) || (dst.orig_val != dst.val)) {
- switch (dst.type) {
- case OP_REG:
- /* The 4-byte case *is* correct: in 64-bit mode we zero-extend. */
- switch (dst.bytes) {
- case 1:
- *(u8 *)dst.ptr = (u8)dst.val;
- break;
- case 2:
- *(u16 *)dst.ptr = (u16)dst.val;
- break;
- case 4:
- *dst.ptr = (u32)dst.val;
- break; /* 64b: zero-ext */
- case 8:
- *dst.ptr = dst.val;
- break;
- }
+ switch (dst.type) {
+ case OP_REG:
+ /* The 4-byte case *is* correct: in 64-bit mode we zero-extend. */
+ switch (dst.bytes) {
+ case 1:
+ *(u8 *)dst.ptr = (u8)dst.val;
break;
- case OP_MEM:
- if (lock_prefix)
- rc = ops->cmpxchg_emulated((unsigned long)dst.
- ptr, &dst.orig_val,
- &dst.val, dst.bytes,
- ctxt);
- else
- rc = ops->write_emulated((unsigned long)dst.ptr,
- &dst.val, dst.bytes,
- ctxt);
- if (rc != 0)
- goto done;
- default:
+ case 2:
+ *(u16 *)dst.ptr = (u16)dst.val;
+ break;
+ case 4:
+ *dst.ptr = (u32)dst.val;
+ break; /* 64b: zero-ext */
+ case 8:
+ *dst.ptr = dst.val;
break;
}
+ break;
+ case OP_MEM:
+ if (lock_prefix)
+ rc = ops->cmpxchg_emulated((unsigned long)dst.
+ ptr, &dst.orig_val,
+ &dst.val, dst.bytes,
+ ctxt);
+ else
+ rc = ops->write_emulated((unsigned long)dst.ptr,
+ &dst.val, dst.bytes,
+ ctxt);
+ if (rc != 0)
+ goto done;
+ default:
+ break;
}

/* Commit shadow register state. */
--- a/kernel/kvm_main.c 2007-06-15 21:18:08.000000000 +0200
+++ b/kernel/kvm_main.c 2007-06-17 16:59:33.000000000 +0200
@@ -1139,8 +1139,10 @@
return 0;
mark_page_dirty(vcpu->kvm, gpa >> PAGE_SHIFT);
virt = kmap_atomic(page, KM_USER0);
- kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
- memcpy(virt + offset_in_page(gpa), val, bytes);
+ if (memcmp(virt + offset_in_page(gpa), val, bytes)) {
+ kvm_mmu_pte_write(vcpu, gpa, virt + offset, val, bytes);
+ memcpy(virt + offset_in_page(gpa), val, bytes);
+ }
kunmap_atomic(virt, KM_USER0);
return 1;
}


Luca
--
Runtime error 6D at f000:a12f : user incompetente
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