Re: [Qemu-devel] [PATCH v2 1/9] block: Add COR filter driver

[Max Reitz <mreitz@redhat.com>]

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On 2018-04-24 17:08, Alberto Garcia wrote:
> On Sat 21 Apr 2018 03:29:21 PM CEST, Max Reitz wrote:
>> This adds a simple copy-on-read filter driver.  It relies on the already
>> existing COR functionality in the central block layer code, which may be
>> moved here once we no longer need it there.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
> 
> 
>> +#define PERM_PASSTHROUGH (BLK_PERM_CONSISTENT_READ \
>> +                          | BLK_PERM_WRITE \
>> +                          | BLK_PERM_RESIZE)
>> +#define PERM_UNCHANGED (BLK_PERM_ALL & ~PERM_PASSTHROUGH)
>> +
>> +static void cor_child_perm(BlockDriverState *bs, BdrvChild *c,
>> +                           const BdrvChildRole *role,
>> +                           BlockReopenQueue *reopen_queue,
>> +                           uint64_t perm, uint64_t shared,
>> +                           uint64_t *nperm, uint64_t *nshared)
>> +{
>> +    if (c == NULL) {
>> +        *nperm = (perm & PERM_PASSTHROUGH) | BLK_PERM_WRITE_UNCHANGED;
>> +        *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
>> +        return;
>> +    }
>> +
>> +    *nperm = (perm & PERM_PASSTHROUGH) |
>> +             (c->perm & PERM_UNCHANGED);
> 
> I admit I'm not completely familiar with this, but don't you need to add
> BLK_PERM_WRITE_UNCHANGED to *nperm ?

As long as it's requested in when the child is attached (which it is in
the "c == NULL" case), it should be part of c->perm then.

(And since PERM_PASSTHROUGH does not contain WRITE_UNCHANGED, it is part
of PERM_UNCHANGED.)

Max

>> +    *nshared = (shared & PERM_PASSTHROUGH) |
>> +               (c->shared_perm & PERM_UNCHANGED);
>> +}
> 
> Berto
> 



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