Re: [PATCH v3 06/19] xen/arm: mm: Avoid flushing the TLBs when mapping are inserted

[Julien Grall <julien@xen.org>]

Hi Stefano,

On 02/04/2022 01:00, Stefano Stabellini wrote:
> On Mon, 21 Feb 2022, Julien Grall wrote:
>> From: Julien Grall <jgrall@amazon.com>
>>
>> Currently, the function xen_pt_update() will flush the TLBs even when
>> the mappings are inserted. This is a bit wasteful because we don't
>> allow mapping replacement. Even if we were, the flush would need to
>> happen earlier because mapping replacement should use Break-Before-Make
>> when updating the entry.
>>
>> A single call to xen_pt_update() can perform a single action. IOW, it
>> is not possible to, for instance, mix inserting and removing mappings.
>> Therefore, we can use `flags` to determine what action is performed.
>>
>> This change will be particularly help to limit the impact of switching
>> boot time mapping to use xen_pt_update().
>>
>> Signed-off-by: Julien Grall <jgrall@amazon.com>
>>
>> ---
>>      Changes in v2:
>>          - New patch
>> ---
>>   xen/arch/arm/mm.c | 17 ++++++++++++++---
>>   1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
>> index fd16c1541ce2..7b4b9de8693e 100644
>> --- a/xen/arch/arm/mm.c
>> +++ b/xen/arch/arm/mm.c
>> @@ -1104,7 +1104,13 @@ static bool xen_pt_check_entry(lpae_t entry, mfn_t mfn, unsigned int level,
>>           /* We should be here with a valid MFN. */
>>           ASSERT(!mfn_eq(mfn, INVALID_MFN));
>>   
>> -        /* We don't allow replacing any valid entry. */
>> +        /*
>> +         * We don't allow replacing any valid entry.
>> +         *
>> +         * Note that the function xen_pt_update() relies on this
>> +         * assumption and will skip the TLB flush. The function will need
>> +         * to be updated if the check is relaxed.
>> +         */
>>           if ( lpae_is_valid(entry) )
>>           {
>>               if ( lpae_is_mapping(entry, level) )
>> @@ -1417,11 +1423,16 @@ static int xen_pt_update(unsigned long virt,
>>       }
>>   
>>       /*
>> -     * Flush the TLBs even in case of failure because we may have
>> +     * The TLBs flush can be safely skipped when a mapping is inserted
>> +     * as we don't allow mapping replacement (see xen_pt_check_entry()).
>> +     *
>> +     * For all the other cases, the TLBs will be flushed unconditionally
>> +     * even if the mapping has failed. This is because we may have
>>        * partially modified the PT. This will prevent any unexpected
>>        * behavior afterwards.
>>        */
>> -    flush_xen_tlb_range_va(virt, PAGE_SIZE * nr_mfns);
>> +    if ( !(flags & _PAGE_PRESENT) || mfn_eq(mfn, INVALID_MFN) )
>> +        flush_xen_tlb_range_va(virt, PAGE_SIZE * nr_mfns);
> 
> I am trying to think of a care where the following wouldn't be enough
> but I cannot come up with one:
> 
>     if ( mfn_eq(mfn, INVALID_MFN) )
>         flush_xen_tlb_range_va(virt, PAGE_SIZE * nr_mfns);

_PAGE_PRESENT is not set for two cases: when removing a page or 
populating page-tables for a region. Both of them will expect an 
INVALID_MFN (see the two asserts in xen_pt_check_entry()).

Therefore your solution should work. However, technically the 'mfn' is 
ignored in both situation (hence why this is an ASSERT() rather than a 
prod check).

Also, I feel it is better to flush more than less (missing a flush could 
have catastrophic result). So I chose to be explicit in which case the 
flush can be skipped.

Maybe it would be clearer if I write:

  !((flags & _PAGE_PRESENT) && !mfn_eq(mfn, INVALID_MFN))

Cheers,

-- 
Julien Grall