Re: [Xen-devel] [PATCH RFC 2/4] xen: grant_table: implement grant_table_soft_reset()

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

"Jan Beulich" <JBeulich@xxxxxxxx> writes:

>>>> On 03.06.15 at 15:35, <vkuznets@xxxxxxxxxx> wrote:
>> When soft reset is being performed we need to replace all actively
>> granted pages with empty pages to prevent possible future memory
>> corruption as the newly started kernel won't be aware of these
>> granted pages.
>> 
>> We make the tot_pages < max_pages assumption here: previously granted pages
>> need to belong to someone and we don't want to implement possible DoS by
>> reassigning them to the grantee/anonymous domain/xen/.. (the malicious guest
>> will be able to consume all host's memory).
>
> How is that going to look in practice? I.e. won't this cause frequent
> failures?
>

I'm not sure we actually need that in practice. In my testing backends
(even with persistent grants enabled) collaborate nicely and release all
grants. I can see a single page still being held and I suppose it's
being held by QEMU (haven't checked what that but I think it is the
console ring). In case we go for the toolstack-assisted approach we can
restart qemu and add some warning when there are active grants.

-- 
  Vitaly

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