Re: [Xen-devel] [PATCH v2 11/25] arm/altp2m: Add HVMOP_altp2m_destroy_p2m.

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

On 08/06/2016 03:36 PM, Julien Grall wrote:
>
>
> On 06/08/2016 10:54, Sergej Proskurin wrote:
>> Hi Julien,
>
> Hello Sergej,
>
>> On 08/04/2016 01:46 PM, Julien Grall wrote:
>>> Hello Sergej,
>>>
>>> On 01/08/16 18:10, Sergej Proskurin wrote:
>>>> Signed-off-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
>>>> ---
>>>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>>> Cc: Julien Grall <julien.grall@xxxxxxx>
>>>> ---
>>>> v2: Substituted the call to tlb_flush for p2m_flush_table.
>>>>     Added comments.
>>>>     Cosmetic fixes.
>>>> ---
>>>>  xen/arch/arm/altp2m.c        | 50
>>>> ++++++++++++++++++++++++++++++++++++++++++++
>>>>  xen/arch/arm/hvm.c           |  2 +-
>>>>  xen/include/asm-arm/altp2m.h |  4 ++++
>>>>  3 files changed, 55 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/xen/arch/arm/altp2m.c b/xen/arch/arm/altp2m.c
>>>> index c22d2e4..80ed553 100644
>>>> --- a/xen/arch/arm/altp2m.c
>>>> +++ b/xen/arch/arm/altp2m.c
>
> [...]
>
>>>> +
>>>> +            p2m_flush_table(p2m);
>>>> +
>>>> +            /*
>>>> +             * Reset VTTBR.
>>>> +             *
>>>> +             * Note that VMID is not freed so that it can be reused
>>>> later.
>>>> +             */
>>>> +            p2m->vttbr.vttbr = INVALID_VTTBR;
>>>> +            d->arch.altp2m_vttbr[idx] = INVALID_VTTBR;
>>>> +
>>>> +            read_unlock(&p2m->lock);
>>>
>>> Why did you decide to reset the p2m rather than free it? The code
>>> would be simpler with the latter.
>>>
>>
>> * First, to be as similar to the x86 implementation as possible.
>> * Second, simply to reuse already allocated p2m's without additional
>> overhead.
>
> I don't really buy those arguments. The x86 implementation may not fit
> the ARM model. Initializing a p2m on ARM is very quick: allocating the
> p2m and the root pages (up to 2 page).
>
> So the overhead is really minimal. Therefore, I don't see any reason
> to introduce complex bookkeeping when NULL can be used for the same
> purpose.
>

Alright, I will adjust the implementation accordingly. Thank you.

Best regards,
~Sergej

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