Re: [kvm-devel] performance with guests running 2.4 kernels (specifically RHEL3)

[Avi Kivity <avi@qumranet.com>]

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David S. Ahern wrote:
> Avi:
>
> We did not get a chance to do this at the Forum. I'd be interested in
> whatever options you have for reducing the scan time further (e.g., try
> to get scan time down to 1-2 seconds).
>
>   

I'm unlikely to get time to do this properly for at least a week, as 
this will be quite difficult and I'm already horribly backlogged.  
However there's an alternative option, modifying the source and getting 
it upstreamed, as I think RHEL 3 is still maintained.

The attached patch (untested) should give a 3X boost for kmap_atomics, 
by folding the two accesses to set the pte into one, and by dropping the 
access that clears the pte.  Unfortunately it breaks the ABI, since 
external modules will inline the original kmap_atomic() which expects 
the pte to be cleared.

This can be worked around by allocating new fixmap slots for kmap_atomic 
with the new behavior, and keeping the old slots with the old behavior, 
but we should first see if the maintainers are open to performance 
optimizations targeting kvm.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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--- include/asm-i386/atomic_kmap.h.orig	2007-06-12 00:24:29.000000000 +0300
+++ include/asm-i386/atomic_kmap.h	2008-06-22 09:23:26.000000000 +0300
@@ -51,18 +51,13 @@ static inline void *__kmap_atomic(struct
 
 	idx = type + KM_TYPE_NR*smp_processor_id();
 	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
-#if HIGHMEM_DEBUG
-	if (!pte_none(*(kmap_pte-idx)))
-		out_of_line_bug();
-#else
 	/*
 	 * Performance optimization - do not flush if the new
 	 * pte is the same as the old one:
 	 */
 	if (pte_val(*(kmap_pte-idx)) == pte_val(mk_pte(page, kmap_prot)))
 		return (void *) vaddr;
-#endif
-	set_pte(kmap_pte-idx, mk_pte(page, kmap_prot));
+	set_pte_atomic(kmap_pte-idx, mk_pte(page, kmap_prot));
 	__flush_tlb_one(vaddr);
 
 	return (void*) vaddr;
@@ -77,12 +72,6 @@ static inline void __kunmap_atomic(void 
 	if (vaddr != __fix_to_virt(FIX_KMAP_BEGIN+idx))
 		out_of_line_bug();
 
-	/*
-	 * force other mappings to Oops if they'll try to access
-	 * this pte without first remap it
-	 */
-	pte_clear(kmap_pte-idx);
-	__flush_tlb_one(vaddr);
 #endif
 }