Re: [PATCH] x86: cpu: Error out if memory exceeds addressable range

[Nikunj A. Dadhania]

On 7/18/2022 7:15 PM, Joao Martins wrote:
> On 7/18/22 14:10, Nikunj A. Dadhania wrote:
>> On 7/18/2022 6:12 PM, Igor Mammedov wrote:
>>> On Mon, 18 Jul 2022 13:47:34 +0530
>>> Nikunj A Dadhania <nikunj@amd.com> wrote:
>>>
>>>> Currently it is possible to start a guest with memory that is beyond
>>>> the addressable range of CPU and QEMU does not even warn about it.
>>>> The default phys_bits is 40 and can address 1TB. However it allows to
>>>> start a guest with greater than 1TB memory.
>>>>
>>>> Prevent this by erroring out in such a scenario.
>>>>
>>>> Reported-by: Shaju Abraham <Abraham.Shaju@amd.com>
>>>> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
>>>
>>>
>>> Following shall care of your issue:
>>> https://www.mail-archive.com/qemu-devel@nongnu.org/msg900136.html
>>
>> Thanks, I tried out the patch series, I could start guest till 978G (not 
>> sure 
>> why this magic number yet) and after that I start getting errors:
> 
> It's expected. The point of the series is meant to avoid attempting at DMA 
> mapping
> over the HyperTransport region. Before it would just fail to either 
> hotplug/boot with VFIO
> devices on kernels >= 5.4 (even if older kernels or other configs let you go 
> through you
> might still see IOMMU errors at some point). So what we essentially do is to 
> have the
> region above 4G to instead start at 1T, thus requiring 1 more phys-bit on 
> cases like this
> where the max gpa hits the Hyper Transport reserved region.
> 
> The cover-letter and this patch
> (20220715171628.21437-11-joao.m.martins@oracle.com/">https://lore.kernel.org/qemu-devel/20220715171628.21437-11-joao.m.martins@oracle.com/
> should clarify on the logic.

Thanks looks good !

> The check you're adding here is essentially patch 9 of the series.

Yes, saw that change.

Regards
Nikunj