Re: KVM x86: Infinite loop on updating accessed bit in r/o page table

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2014-02-25 11:57, Paolo Bonzini wrote:
> Il 24/02/2014 19:29, Jan Kiszka ha scritto:
>> Hi,
>>
>> I noticed that KVM (with VMX at least) enters an inifite loop of
>> vmentries and ept-violations when it has to set the accessed bit in a
>> guest page table that is in read-only memory (namely: the F-segment of
>> the BIOS). I don't think this is the proper reaction...
>>
>> Jan
> 
> Thanks, I'll try to reproduce this.  Does it work with shadow page tables?

It gets beyond the point where Accessed should be set but loops over
trying to update Dirty in the page table. QEMU in emulation mode simply
ignores both write attempts. I think our ROM should behave like this,
i.e. simply swallow writes.

> I'm asking because this commit wanted to fix something similar for the shadow
> page table case:
> 
>     commit ba6a3541545542721ce821d1e7e5ce35752e6fdf
>     Author: Paolo Bonzini <pbonzini@redhat.com>
>     Date:   Mon Sep 9 13:52:33 2013 +0200
> 
>     KVM: mmu: allow page tables to be in read-only slots
>     
>     Page tables in a read-only memory slot will currently cause a triple
>     fault because the page walker uses gfn_to_hva and it fails on such a slot.
>     
>     OVMF uses such a page table; however, real hardware seems to be fine with
>     that as long as the accessed/dirty bits are set.  Save whether the slot
>     is readonly, and later check it when updating the accessed and dirty bits.
>     
>     Reviewed-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
>     Reviewed-by: Gleb Natapov <gleb@redhat.com>
>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> even though OVMF sets the accessed/dirty bits so it's not exactly the same
> scenario.
> 
> Note that NPT simply does not support this.  Page tables must be writable
> in the NPT page tables, according to the AMD manual.

We could still emulate the instruction in such a case to get beyond the
write fault. Slow but more correct, I suppose.

Jan

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