* Chirantan Ekbote ([email protected]) wrote:
> On Wed, Feb 10, 2021 at 4:04 AM Dr. David Alan Gilbert (git)
> <[email protected]> wrote:
> >
> > From: "Dr. David Alan Gilbert" <[email protected]>
> > +
> > +typedef struct {
> > + /* Offsets within the file being mapped */
> > + uint64_t fd_offset[VHOST_USER_FS_SLAVE_ENTRIES];
> > + /* Offsets within the cache */
> > + uint64_t c_offset[VHOST_USER_FS_SLAVE_ENTRIES];
> > + /* Lengths of sections */
> > + uint64_t len[VHOST_USER_FS_SLAVE_ENTRIES];
> > + /* Flags, from VHOST_USER_FS_FLAG_* */
> > + uint64_t flags[VHOST_USER_FS_SLAVE_ENTRIES];
> > +} VhostUserFSSlaveMsg;
> > +
>
> Is it too late to change this? This struct allocates space for up to
> 8 entries but most of the time the server will only try to set up one
> mapping at a time so only 32 out of the 256 bytes in the message are
> actually being used. We're just wasting time memcpy'ing bytes that
> will never be used. Is there some reason this can't be dynamically
> sized? Something like:
>
> typedef struct {
> /* Number of mapping requests */
> uint16_t num_requests;
> /* `num_requests` mapping requests */
> MappingRequest requests[];
> } VhostUserFSSlaveMsg;
>
> typedef struct {
> /* Offset within the file being mapped */
> uint64_t fd_offset;
> /* Offset within the cache */
> uint64_t c_offset;
> /* Length of section */
> uint64_t len;
> /* Flags, from VHOST_USER_FS_FLAG_* */
> uint64_t flags;
> } MappingRequest;
>
> The current pre-allocated structure both wastes space when there are
> fewer than 8 requests and requires extra messages to be sent when
> there are more than 8 requests. I realize that in the grand scheme of
> things copying 224 extra bytes is basically not noticeable but it just
> irks me that we could fix this really easily before it gets propagated
> to too many other places.
So this has come out as:
typedef struct {
/* Offsets within the file being mapped */
uint64_t fd_offset;
/* Offsets within the cache */
uint64_t c_offset;
/* Lengths of sections */
uint64_t len;
/* Flags, from VHOST_USER_FS_FLAG_* */
uint64_t flags;
} VhostUserFSSlaveMsgEntry;
typedef struct {
/* Generic flags for the overall message */
uint32_t flags;
/* Number of entries */
uint16_t count;
/* Spare */
uint16_t align;
VhostUserFSSlaveMsgEntry entries[];
} VhostUserFSSlaveMsg;
which seems to work OK.
I've still got a:
#define VHOST_USER_FS_SLAVE_MAX_ENTRIES 8
to limit the size VhostUserFSSlaveMsg can get to.
The variable length array makes the union in the reader a bit more
hairy, but it's OK.
Dave
> Chirantan
>
> > --
> > 2.29.2
> >
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> >
>
--
Dr. David Alan Gilbert / [email protected] / Manchester, UK
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