Re: [PATCH][4.17] EFI: don't convert memory marked for runtime use to ordinary RAM

From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>,
	Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Volodymyr Babchuk <volodymyr_babchuk@epam.com>,
	Bertrand Marquis <bertrand.marquis@arm.com>,
	Henry Wang <Henry.Wang@arm.com>
Subject: Re: [PATCH][4.17] EFI: don't convert memory marked for runtime use to ordinary RAM
Date: Tue, 4 Oct 2022 14:18:31 +0200	[thread overview]
Message-ID: <9aa0569a-bc18-9480-c86a-817222d4fea1@suse.com> (raw)
In-Reply-To: <YzwQymW3jkfzUQrI@MacBook-Air-de-Roger.local>

On 04.10.2022 12:54, Roger Pau Monné wrote:
> On Tue, Oct 04, 2022 at 12:44:16PM +0200, Jan Beulich wrote:
>> On 04.10.2022 12:38, Roger Pau Monné wrote:
>>> On Tue, Oct 04, 2022 at 12:23:23PM +0200, Jan Beulich wrote:
>>>> On 04.10.2022 11:33, Roger Pau Monné wrote:
>>>>> On Tue, Oct 04, 2022 at 10:06:36AM +0200, Jan Beulich wrote:
>>>>>> On 30.09.2022 16:28, Roger Pau Monné wrote:
>>>>>>> On Fri, Sep 30, 2022 at 09:50:40AM +0200, Jan Beulich wrote:
>>>>>>>> efi_init_memory() in both relevant places is treating EFI_MEMORY_RUNTIME
>>>>>>>> higher priority than the type of the range. To avoid accessing memory at
>>>>>>>> runtime which was re-used for other purposes, make
>>>>>>>> efi_arch_process_memory_map() follow suit. While on x86 in theory the
>>>>>>>> same would apply to EfiACPIReclaimMemory, we don't actually "reclaim"
>>>>>>>> E820_ACPI memory there and hence that type's handling can be left alone.
>>>>>>>
>>>>>>> What about dom0?  Should it be translated to E820_RESERVED so that
>>>>>>> dom0 doesn't try to use it either?
>>>>>>
>>>>>> I'm afraid I don't understand the questions. Not the least because I
>>>>>> think "it" can't really mean "dom0" from the earlier sentence.
>>>>>
>>>>> Sorry, let me try again:
>>>>>
>>>>> The memory map provided to dom0 will contain E820_ACPI entries for
>>>>> memory ranges with the EFI_MEMORY_RUNTIME attributes in the EFI memory
>>>>> map.  Is there a risk from dom0 reclaiming such E820_ACPI ranges,
>>>>> overwriting the data needed for runtime services?
>>>>
>>>> How would Dom0 go about doing so? It has no control over what we hand
>>>> to the page allocator - it can only free pages which were actually
>>>> allocated to it. E820_ACPI and E820_RESERVED pages are assigned to
>>>> DomIO - Dom0 can map and access them, but it cannot free them.
>>>
>>> Maybe I'm very confused, but what about dom0 overwriting the data
>>> there, won't it cause issues to runtime services?
>>
>> If it overwrites it, of course there are going to be issues. Just like
>> there are going to be problems from anything else Dom0 does wrong.
> 
> But would dom0 know it's doing something wrong?

Yes. Please also see my reply to Andrew.

> The region is just marked as E820_ACPI from dom0 PoV, so it doesn't
> know it's required by EFI runtime services, and dom0 could
> legitimately overwrite the region once it considers all ACPI parsing
> done from it's side.

PV Dom0 won't ever see E820_ACPI in the relevant E820 map; this type can
only appear in the machine E820. In how far PVH Dom0 might need to take
special care I can't tell right now (but at least for kexec purposes I
expect Linux isn't going to recycle E820_ACPI regions even going forward).

Jan