Re: [Xen-devel] [PATCH] Fix shared entry status for grant copy operation on paged out gfn

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

On Aug 22, 2012, at 10:21 AM, Jan Beulich wrote:

>>>> On 21.08.12 at 22:13, Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx> wrote:
>> xen/common/grant_table.c |  33 ++++++++++++++++++++++-----------
>> 1 files changed, 22 insertions(+), 11 deletions(-)
>> 
>> 
>> The unwind path was not clearing the shared entry status bits. This was
>> BSOD-ing guests on network activity under certain configurations.
>> 
>> Also:
>> * sed the fixup method name to signal it's related to grant copy.
>> * use atomic clear flag ops during fixup.
> 
> Is that last thing really needed? I remember having looked at
> these non-atomic operations too a little while back, and came to
> the conclusion that probably the authors intentionally coded it
> that way.

Due to some obscure property of transitive grants? All other flag 
clearing/setting in shared grant entries by the hypervisor is done atomically 
(GTF_transfer_completed being an exception).

>From my p.o.v. there is no downside. But I am not 100% certain and I can back 
>it off.

Thanks
Andres

> 
> jan
> 
>> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
>> 
>> diff -r 5267f78c8a1e -r 84a23ae853a5 xen/common/grant_table.c
>> --- a/xen/common/grant_table.c
>> +++ b/xen/common/grant_table.c
>> @@ -1751,14 +1751,14 @@ __release_grant_for_copy(
>>    under the domain's grant table lock. */
>> /* Only safe on transitive grants.  Even then, note that we don't
>>    attempt to drop any pin on the referent grant. */
>> -static void __fixup_status_for_pin(const struct active_grant_entry *act,
>> +static void __fixup_status_for_copy_pin(const struct active_grant_entry 
>> *act,
>>                                    uint16_t *status)
>> {
>>     if ( !(act->pin & GNTPIN_hstw_mask) )
>> -        *status &= ~GTF_writing;
>> +        gnttab_clear_flag(_GTF_writing, status);
>> 
>>     if ( !(act->pin & GNTPIN_hstr_mask) )
>> -        *status &= ~GTF_reading;
>> +        gnttab_clear_flag(_GTF_reading, status);
>> }
>> 
>> /* Grab a frame number from a grant entry and update the flags and pin
>> @@ -1834,7 +1834,7 @@ __acquire_grant_for_copy(
>>         if ( sha2 && (shah->flags & GTF_type_mask) == GTF_transitive )
>>         {
>>             if ( !allow_transitive )
>> -                PIN_FAIL(unlock_out, GNTST_general_error,
>> +                PIN_FAIL(unlock_out_clear, GNTST_general_error,
>>                          "transitive grant when transitivity not 
>> allowed\n");
>> 
>>             trans_domid = sha2->transitive.trans_domid;
>> @@ -1842,7 +1842,7 @@ __acquire_grant_for_copy(
>>             barrier(); /* Stop the compiler from re-loading
>>                           trans_domid from shared memory */
>>             if ( trans_domid == rd->domain_id )
>> -                PIN_FAIL(unlock_out, GNTST_general_error,
>> +                PIN_FAIL(unlock_out_clear, GNTST_general_error,
>>                          "transitive grants cannot be self-referential\n");
>> 
>>             /* We allow the trans_domid == ldom case, which
>> @@ -1855,7 +1855,7 @@ __acquire_grant_for_copy(
>>             /* We need to leave the rrd locked during the grant copy */
>>             td = rcu_lock_domain_by_id(trans_domid);
>>             if ( td == NULL )
>> -                PIN_FAIL(unlock_out, GNTST_general_error,
>> +                PIN_FAIL(unlock_out_clear, GNTST_general_error,
>>                          "transitive grant referenced bad domain %d\n",
>>                          trans_domid);
>>             spin_unlock(&rgt->lock);
>> @@ -1866,7 +1866,7 @@ __acquire_grant_for_copy(
>> 
>>             spin_lock(&rgt->lock);
>>             if ( rc != GNTST_okay ) {
>> -                __fixup_status_for_pin(act, status);
>> +                __fixup_status_for_copy_pin(act, status);
>>                 rcu_unlock_domain(td);
>>                 spin_unlock(&rgt->lock);
>>                 return rc;
>> @@ -1878,7 +1878,7 @@ __acquire_grant_for_copy(
>>                and try again. */
>>             if ( act->pin != old_pin )
>>             {
>> -                __fixup_status_for_pin(act, status);
>> +                __fixup_status_for_copy_pin(act, status);
>>                 rcu_unlock_domain(td);
>>                 spin_unlock(&rgt->lock);
>>                 put_page(*page);
>> @@ -1897,7 +1897,7 @@ __acquire_grant_for_copy(
>>         {
>>             rc = __get_paged_frame(sha1->frame, &grant_frame, page, 
>> readonly, 
>> rd);
>>             if ( rc != GNTST_okay )
>> -                goto unlock_out;
>> +                goto unlock_out_clear;
>>             act->gfn = sha1->frame;
>>             is_sub_page = 0;
>>             trans_page_off = 0;
>> @@ -1907,7 +1907,7 @@ __acquire_grant_for_copy(
>>         {
>>             rc = __get_paged_frame(sha2->full_page.frame, &grant_frame, 
>> page, 
>> readonly, rd);
>>             if ( rc != GNTST_okay )
>> -                goto unlock_out;
>> +                goto unlock_out_clear;
>>             act->gfn = sha2->full_page.frame;
>>             is_sub_page = 0;
>>             trans_page_off = 0;
>> @@ -1917,7 +1917,7 @@ __acquire_grant_for_copy(
>>         {
>>             rc = __get_paged_frame(sha2->sub_page.frame, &grant_frame, page, 
>> readonly, rd);
>>             if ( rc != GNTST_okay )
>> -                goto unlock_out;
>> +                goto unlock_out_clear;
>>             act->gfn = sha2->sub_page.frame;
>>             is_sub_page = 1;
>>             trans_page_off = sha2->sub_page.page_off;
>> @@ -1948,6 +1948,17 @@ __acquire_grant_for_copy(
>>     *length = act->length;
>>     *frame = act->frame;
>> 
>> +    spin_unlock(&rgt->lock);
>> +    return rc;
>> + 
>> + unlock_out_clear:
>> +    if ( !(readonly) &&
>> +         !(act->pin & GNTPIN_hstw_mask) )
>> +        gnttab_clear_flag(_GTF_writing, status);
>> +
>> +    if ( !act->pin )
>> +        gnttab_clear_flag(_GTF_reading, status);
>> +
>>  unlock_out:
>>     spin_unlock(&rgt->lock);
>>     return rc;
> 
> 


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