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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 08/14/2012 01:39 AM, Marcelo Tosatti wrote:
> 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?
> 

You can get the discussion at:
https://lkml.org/lkml/2012/5/22/228

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

My fault. :(

> 
>>> 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.
> 

That is exactly what i did in the first version. :)

You can see it and the reason why it switched to the new way (readonly memslot)
in the above website (the first message in thread).

> 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.

Sorry, Why? After re-entry to the guest, it can not generate a correct MMIO?