Re: ABAT Testing report, kernel 76f0301b5.., user 04cff0e47..

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Zhao, Yunfeng wrote:
> Hi, all
>
> This is today's ABAT testing result against kvm.git 76f0301b5e4d2603d8e1ee5295db29faea660b49 and kvm-userspace.git 04cff0e47c4cc1e3d51f917a657b97ac2ed29719
> We are still using UP guests in the testing, we will try to enable some SMP Windows and Linux tests in this week.
>
> One new issue is about performance regression since KVM-29
> 1. Performance regression since KVM-29
> https://sourceforge.net/tracker/index.php?func=detail&aid=1758756&group_id=180599&atid=893831
>
>   

I bisected this to

commit 8a1449563b3e5ede56b28cc977c8da22a17cdf51
Author: Luca Tettamanti <kronos.it-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Date:   Tue Jun 19 22:41:38 2007 +0200

    KVM: Avoid useless memory write when possible

    When writing to normal memory and the memory area is unchanged the write
    can be safely skipped, avoiding the costly kvm_mmu_pte_write.

    Signed-Off-By: Luca Tettamanti <kronos.it-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
    Signed-off-by: Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index 7826f16..5603000 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -1170,8 +1170,10 @@ static int emulator_write_phys(struct kvm_vcpu 
*vcpu, gpa_t gpa,
                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;
 }

The effect with this patch applied is that pf_fixed grows very high.  
I'm guessing that this is because not calling kvm_mmu_pte_write() 
defeats the fork detector and causes a high amount of emulated writes to 
page table when fork() resets the writable bit on read-only ptes.

-- 
error compiling committee.c: too many arguments to function


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