On 5/20/19 4:35 PM, Farhan Ali wrote:
>
>
> On 05/16/2019 12:14 PM, Eric Farman wrote:
>> The skip flag of a CCW offers the possibility of data not being
>> transferred, but is only meaningful for certain commands.
>> Specifically, it is only applicable for a read, read backward, sense,
>> or sense ID CCW and will be ignored for any other command code
>> (SA22-7832-11 page 15-64, and figure 15-30 on page 15-75).
>>
>> (A sense ID is xE4, while a sense is x04 with possible modifiers in the
>> upper four bits. So we will cover the whole "family" of sense CCWs.)
>>
>> For those scenarios, since there is no requirement for the target
>> address to be valid, we should skip the call to vfio_pin_pages() and
>> rely on the IDAL address we have allocated/built for the channel
>> program. The fact that the individual IDAWs within the IDAL are
>> invalid is fine, since they aren't actually checked in these cases.
>>
>> Set pa_nr to zero when skipping the pfn_array_pin() call, since it is
>> defined as the number of pages pinned and is used to determine
>> whether to call vfio_unpin_pages() upon cleanup.
>>
>> As we do this, since the pfn_array_pin() routine returns the number of
>> pages pinned, and we might not be doing that, the logic for converting
>> a CCW from direct-addressed to IDAL needs to ensure there is room for
>> one IDAW in the IDAL being built since a zero-length IDAL isn't great.
>>
>> Signed-off-by: Eric Farman<farman@xxxxxxxxxxxxx>
>> ---
>> drivers/s390/cio/vfio_ccw_cp.c | 55 ++++++++++++++++++++++++++++++----
>> 1 file changed, 50 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/s390/cio/vfio_ccw_cp.c
>> b/drivers/s390/cio/vfio_ccw_cp.c
>> index 086faf2dacd3..0467838aed23 100644
>> --- a/drivers/s390/cio/vfio_ccw_cp.c
>> +++ b/drivers/s390/cio/vfio_ccw_cp.c
>> @@ -294,6 +294,10 @@ static long copy_ccw_from_iova(struct
>> channel_program *cp,
>> /*
>> * Helpers to operate ccwchain.
>> */
>> +#define ccw_is_read(_ccw) (((_ccw)->cmd_code & 0x03) == 0x02)
>> +#define ccw_is_read_backward(_ccw) (((_ccw)->cmd_code & 0x0F) == 0x0C)
>> +#define ccw_is_sense(_ccw) (((_ccw)->cmd_code & 0x0F) ==
>> CCW_CMD_BASIC_SENSE)
>> +
>> #define ccw_is_test(_ccw) (((_ccw)->cmd_code & 0x0F) == 0)
>> #define ccw_is_noop(_ccw) ((_ccw)->cmd_code == CCW_CMD_NOOP)
>> @@ -301,10 +305,39 @@ static long copy_ccw_from_iova(struct
>> channel_program *cp,
>> #define ccw_is_tic(_ccw) ((_ccw)->cmd_code == CCW_CMD_TIC)
>> #define ccw_is_idal(_ccw) ((_ccw)->flags & CCW_FLAG_IDA)
>> -
>> +#define ccw_is_skip(_ccw) ((_ccw)->flags & CCW_FLAG_SKIP)
>> #define ccw_is_chain(_ccw) ((_ccw)->flags & (CCW_FLAG_CC |
>> CCW_FLAG_DC))
>> +/*
>> + * ccw_does_data_transfer()
>> + *
>> + * Determine whether a CCW will move any data, such that the guest pages
>> + * would need to be pinned before performing the I/O.
>> + *
>> + * Returns 1 if yes, 0 if no.
>> + */
>> +static inline int ccw_does_data_transfer(struct ccw1 *ccw)
>> +{
>> + /* If the skip flag is off, then data will be transferred */
>> + if (!ccw_is_skip(ccw))
>> + return 1;
>> +
>> + /*
>> + * If the skip flag is on, it is only meaningful if the command
>> + * code is a read, read backward, sense, or sense ID. In those
>> + * cases, no data will be transferred.
>> + */
>> + if (ccw_is_read(ccw) || ccw_is_read_backward(ccw))
>> + return 0;
>> +
>> + if (ccw_is_sense(ccw))
>> + return 0;
>
> Just out of curiosity, is there a reason we are checking ccw_is_sense in
> a separate if statement?
No reason besides I thought it read nicer this way, with read
forward/backward being grouped together and not needing to force
everything to fit in 80 columns. Knowing another opcode (NOP) would be
added later made this layout seem logical too.
The generated assembly is identical regardless of how it's written,
which is not surprising based on the different masks that have to be
employed.
- Eric
>
>> +
>> + /* The skip flag is on, but it is ignored for this command code. */
>> + return 1;
>> +}
