Re: BUG at mmu.c:615 from localhost migration using ept+hugetlbfs

[Avi Kivity <avi@redhat.com>]

Marcelo Tosatti wrote:
> Ryan,
>
> On Fri, May 29, 2009 at 11:43:26AM -0500, Ryan Harper wrote:
>   
>> Testing latest qemu-kvm.git and kvm-kmod.git, ept enabled and backing
>> guests with large pages trips a BUG in the mmu code.  If I disable ept,
>> but still use large pages, migration succeeds.  Reproduce with:
>>
>> hugetlbfs setup:
>> % mkdir -p /hugetlbfs && mount -t hugetlbfs hugetlbfs /hugetlbfs
>> % echo 10000 > /proc/sys/vm/nr_hugepages
>>
>> qemu commands:
>>
>> guest a:
>> sudo x86_64-softmmu/qemu-system-x86_64 -L pc-bios -m 2048 -mempath /hugetlbfs -net nic -net tap -vnc :12 -monitor stdio -hda /scratch/images/rharper/rhel4u8-32-ide.raw
>>
>> guest b:
>> sudo x86_64-softmmu/qemu-system-x86_64 -L pc-bios -m 2048 -mempath /hugetlbfs -net nic -net tap -vnc :13 -monitor stdio -hda /scratch/images/rharper/rhel4u8-32-ide.raw -incoming tcp:0:4444
>>
>> Once the guest a is up, issued migrate command:
>> (qemu) migrate -d tcp:localhost:444
>>
>> rmap_remove: ffff880a08e00098 c0336e65c0336e5b 0->BUG
>>     
> 				^^^^^^^^^^^^^^^^
>
> This value looks very strange (bits 5:3 contain invalid value, for one).
> Don't have access to HW at the very moment, so it would be great if you
> had time to do a change equivalent to this and reproduce:
>   

That spte is totally bogus.

> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 809cce0..ceb70b0 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1759,7 +1764,7 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *shadow_pte,
>  			child = page_header(pte & PT64_BASE_ADDR_MASK);
>  			mmu_page_remove_parent_pte(child, shadow_pte);
>  		} else if (pfn != spte_to_pfn(*shadow_pte)) {
> -			pgprintk("hfn old %lx new %lx\n",
> +			printk(KERN_ERR "hfn old %lx new %lx\n",
>  				 spte_to_pfn(*shadow_pte), pfn);
>  			rmap_remove(vcpu->kvm, shadow_pte);
>  		} else
>
> Avi, any hints?
>   

Not really.  One thing, migration should transition the shadow 
pagetables from large pages to small ones, maybe that bit is broken.

Maybe we're looking at a largepage spte and interpreting it as a normal 
L2 spte, and interpreting a guest page as the L1 spt.

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