Re: [Qemu-devel] [RFC PATCH] QEMU may write to system_memory before guest starts

[Юрий Котов]

Ping

21.03.2019, 19:27, "Yury Kotov" <address@hidden>:
> Hi,
>
> 19.03.2019, 14:52, "Dr. David Alan Gilbert" <address@hidden>:
>>  * Peter Maydell (address@hidden) wrote:
>>>   On Tue, 19 Mar 2019 at 11:03, Dr. David Alan Gilbert
>>>   <address@hidden> wrote:
>>>   >
>>>   > * Peter Maydell (address@hidden) wrote:
>>>   > > I didn't think migration distinguished between "main memory"
>>>   > > and any other kind of RAMBlock-backed memory ?
>>>   >
>>>   > In Yury's case there's a distinction between RAMBlock's that are mapped
>>>   > with RAM_SHARED (which normally ends up as MAP_SHARED) and all others.
>>>   > You can set that for main memory by using -numa to specify a memdev
>>>   > that's backed by a file and has the share=on property.
>>>   >
>>>   > On x86 the ROMs end up as separate RAMBlock's that aren't affected
>>>   > by that -numa/share=on - so they don't fight Yury's trick.
>>>
>>>   You can use the generic loader on x86 to load an ELF file
>>>   into RAM if you want, which would I think also trigger this.
>>
>>  OK, although that doesn't worry me too much - since in the majority
>>  of cases Yury's trick still works well.
>>
>>  I wonder if there's a way to make Yury's code to detect these cases
>>  and not allow the feature; the best thing for the moment would seem to
>>  be to skip the aarch test that uses elf loading.
>
> Currently, I've no idea how to detect such cases, but there is an ability to
> detect memory corruption. I want to update the RFC patch to let user to map 
> some
> memory regions as readonly until incoming migration start.
>
> E.g.
> 1) If x-ignore-shared is enabled in command line or memory region is marked
>    (something like ',readonly=on'),
> 2) Memory region is shared (,share=on),
> 3) And qemu is started with '-incoming' option
>
> Then map such regions as readonly until incoming migration finished.
> Thus, the patch will be able to detect memory corruption and will not affect
> normal cases.
>
> How do you think, is it needed?
>
> I already have a cleaner version of the RFC patch, but I'm not sure about 1).
> Which way is better: enable capability in command line, add a new option for
> memory-backend or something else.
>
>>  Dave
>>
>>>   thanks
>>>   -- PMM
>>  --
>>  Dr. David Alan Gilbert / address@hidden / Manchester, UK
>
> Regards,
> Yury