Re: [EXTERNAL] Re: [PATCH] Drivers: hv: Introduce mshv_vtl driver

[Naman Jain <namjain@linux.microsoft.com>]

On 5/13/2025 9:00 PM, Michael Kelley wrote:
> From: Roman Kisel <romank@linux.microsoft.com> Sent: Friday, May 9, 2025 11:05 AM
>>
>> On 5/9/2025 11:02 AM, Saurabh Singh Sengar wrote:
>>>
>>>
>>
>> [...]
>>
>>>> Yep. We don't rely on user land software doing sane things to maintain
>>>> correctness in kernel, so this needs to be fixed.
>>>>
>>>> Thanks,
>>>> Wei.
>>>
>>>
>>> How about fixing this for normal x86 for now and put a TODO for CVM to be fixed
>> later, when we bring in CVM support ?
>>
>> That seems to strike the right balance ihmo :)
>> Thanks for coming up with the suggestion!
>>
> 
> FWIW, it seems like it would be pretty easy to fix the CVM case as well.  Do
> the following:
> 
> 1. Allocate memory at runtime using the normal kmalloc()
> 2. Copy from user space to that allocated memory
> 3. Disable interrupts as usual for using the per-cpu hypercall arg pages
> 4. Copy from the allocated memory to the per-cpu hypercall arg pages.
>     In a CVM this will do the conversion from encrypted memory to
>     decrypted memory.
> 5. Make the hypercall
> 6. Copy out any results to the allocated memory. Again, this will do
>     the conversion from decrypted to encrypted.
> 7. Enable interrupts
> 8. Copy results from the allocated memory to user space
> 9. Free the allocated memory
> 
> (And maybe Steps 6 and 8 don't apply if there's no output data to copy
> back to user space.)
> 
> The performance penalty is the memory allocation/free, plus the extra
> copying of the input/output hypercall arguments. But I'm guessing the
> arguments are usually on the small side, so the extra copy isn't a big issue.
> 
> Michael

Thanks for looking into this Michael. The steps you have highlighted
makes sense. There is an implementation for this here:

https://github.com/microsoft/OHCL-Linux-Kernel/blob/product/hcl-main/6.12/drivers/hv/mshv_vtl_main.c#L2275

Since CVM use-case does not exist as of today in my patch, we thought of
adding this in future patches when we add CVM support.

Regards,
Naman