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

[Yury Kotov]

I saw Catherine Ho's patch series and it seems ok to me, but in this RFC I asked
about a way how to detect other writes which may not be covered by particular
fixes.
Perhaps this is excessive caution...

Regards,
Yury

04.04.2019, 12:52, "Dr. David Alan Gilbert" <address@hidden>:
> * Юрий Котов (address@hidden) wrote:
>>  Ping
>
> Is this fixed by Catherine Ho's patch series?
>
> Dave
>
>>  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
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK