Re: [PATCH v5 00/12] KVM: introduce readonly memslot

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sat, Aug 11, 2012 at 11:36:20AM +0800, Xiao Guangrong wrote:
> On 08/11/2012 02:14 AM, Marcelo Tosatti wrote:
> > On Tue, Aug 07, 2012 at 05:47:15PM +0800, Xiao Guangrong wrote:
> >> Changelog:
> >> - introduce KVM_PFN_ERR_RO_FAULT instead of dummy page
> >> - introduce KVM_HVA_ERR_BAD and optimize error hva indicators
> >>
> >> The test case can be found at:
> >> http://lkml.indiana.edu/hypermail/linux/kernel/1207.2/00819/migrate-perf.tar.bz2
> >>
> >> In current code, if we map a readonly memory space from host to guest
> >> and the page is not currently mapped in the host, we will get a fault-pfn
> >> and async is not allowed, then the vm will crash.
> >>
> >> As Avi's suggestion, We introduce readonly memory region to map ROM/ROMD
> >> to the guest, read access is happy for readonly memslot, write access on
> >> readonly memslot will cause KVM_EXIT_MMIO exit.
> > 
> > Memory slots whose QEMU mapping is write protected is supported
> > today, as long as there are no write faults.
> > 
> > What prevents the use of mmap(!MAP_WRITE) to handle read-only memslots
> > again?
> > 
> 
> It is happy to map !write host memory space to the readonly memslot,
> and they can coexist as well.
> 
> readonly memslot checks the write-permission by seeing slot->flags and
> !write memory checks the write-permission in hva_to_pfn() function
> which checks vma->flags. It is no conflict.

Yes, there is no conflict. The point is, if you can use the
mmap(PROT_READ) interface (supporting read faults on read-only slots)
for this behavior, what is the advantage of a new memslot flag?

I'm not saying mmap(PROT_READ) is the best interface, i am just asking
why it is not.

> > The initial objective was to fix a vm crash, can you explain that
> > initial problem?
> >
> 
> The issue was trigged by this code:
> 
>                 } else {
>                         if (async && (vma->vm_flags & VM_WRITE))
>                                 *async = true;
>                         pfn = KVM_PFN_ERR_FAULT;
>                 }
> 
> If the host memory region is readonly (!vma->vm_flags & VM_WRITE) and
> its physical page is swapped out (or the file data does not be read in),
> get_user_page_nowait will fail, above code reject to set async,
> then we will get a fault pfn and async=false.
> 
> I guess this issue also exists in "QEMU write protected mapping" as
> you mentioned above.

Yes, it does. As far as i understand, what that check does from a high
level pov is:

- Did get_user_pages_nowait() fail due to a swapped out page (in which 
case we should try to swappin the page asynchronously), or due to 
another reason (for which case an error should be returned).

Using vma->vm_flags VM_WRITE for that is trying to guess why
get_user_pages_nowait() failed, because it (gup_nowait return values) 
does not provide sufficient information by itself.

Can't that be fixed separately? 

Another issue which is also present with the mmap(PROT_READ) scheme is
interaction with reexecute_instruction. That is, unless i am mistaken,
reexecute_instruction can succeed (return true) on a region that is
write protected. This breaks the "write faults on read-only slots exit
to userspace via EXIT_MMIO" behaviour.