Re: [PATCH v2 18/42] esp: accumulate SCSI commands for PDMA transfers in cmdbuf instead of pdma_buf

[Laurent Vivier <laurent@vivier.eu>]

Le 02/03/2021 à 20:29, Mark Cave-Ayland a écrit :
> On 02/03/2021 17:59, Laurent Vivier wrote:
> 
>> Le 02/03/2021 à 18:34, Mark Cave-Ayland a écrit :
>>> On 02/03/2021 17:02, Laurent Vivier wrote:
>>>
>>>> Le 09/02/2021 à 20:29, Mark Cave-Ayland a écrit :
>>>>> ESP SCSI commands are already accumulated in cmdbuf and so there is no need to
>>>>> keep a separate pdma_buf buffer. Accumulate SCSI commands for PDMA transfers in
>>>>> cmdbuf instead of pdma_buf so update cmdlen accordingly and change pdma_origin
>>>>> for PDMA transfers to CMD which allows the PDMA origin to be removed.
>>>>>
>>>>> This commit also removes a stray memcpy() from get_cmd() which is a no-op because
>>>>> cmdlen is always zero at the start of a command.
>>>>>
>>>>> Notionally the removal of pdma_buf from vmstate_esp_pdma also breaks migration
>>>>> compatibility for the PDMA subsection until its complete removal by the end of
>>>>> the series.
>>>>>
>>>>> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
>>>>> ---
>>>>>    hw/scsi/esp.c         | 56 +++++++++++++++++++------------------------
>>>>>    include/hw/scsi/esp.h |  2 --
>>>>>    2 files changed, 25 insertions(+), 33 deletions(-)
>>>>>
>>>>> diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
>>>>> index 7134c0aff4..b846f022fb 100644
>>>>> --- a/hw/scsi/esp.c
>>>>> +++ b/hw/scsi/esp.c
>>>>> @@ -139,8 +139,6 @@ static void set_pdma(ESPState *s, enum pdma_origin_id origin,
>>>>>    static uint8_t *get_pdma_buf(ESPState *s)
>>>>>    {
>>>>>        switch (s->pdma_origin) {
>>>>> -    case PDMA:
>>>>> -        return s->pdma_buf;
>>>>>        case TI:
>>>>>            return s->ti_buf;
>>>>>        case CMD:
>>>>> @@ -161,14 +159,12 @@ static uint8_t esp_pdma_read(ESPState *s)
>>>>>        }
>>>>>          switch (s->pdma_origin) {
>>>>> -    case PDMA:
>>>>> -        val = s->pdma_buf[s->pdma_cur++];
>>>>> -        break;
>>>>>        case TI:
>>>>>            val = s->ti_buf[s->pdma_cur++];
>>>>>            break;
>>>>>        case CMD:
>>>>> -        val = s->cmdbuf[s->pdma_cur++];
>>>>> +        val = s->cmdbuf[s->cmdlen++];
>>>>> +        s->pdma_cur++;
>>>>>            break;
>>>>>        case ASYNC:
>>>>>            val = s->async_buf[s->pdma_cur++];
>>>>> @@ -193,14 +189,12 @@ static void esp_pdma_write(ESPState *s, uint8_t val)
>>>>>        }
>>>>>          switch (s->pdma_origin) {
>>>>> -    case PDMA:
>>>>> -        s->pdma_buf[s->pdma_cur++] = val;
>>>>> -        break;
>>>>>        case TI:
>>>>>            s->ti_buf[s->pdma_cur++] = val;
>>>>>            break;
>>>>>        case CMD:
>>>>> -        s->cmdbuf[s->pdma_cur++] = val;
>>>>> +        s->cmdbuf[s->cmdlen++] = val;
>>>>> +        s->pdma_cur++;
>>>>>            break;
>>>>>        case ASYNC:
>>>>>            s->async_buf[s->pdma_cur++] = val;
>>>>> @@ -256,8 +250,7 @@ static uint32_t get_cmd(ESPState *s, uint8_t *buf, uint8_t buflen)
>>>>>            if (s->dma_memory_read) {
>>>>>                s->dma_memory_read(s->dma_opaque, buf, dmalen);
>>>>>            } else {
>>>>> -            memcpy(s->pdma_buf, buf, dmalen);
>>>>> -            set_pdma(s, PDMA, 0, dmalen);
>>>>> +            set_pdma(s, CMD, 0, dmalen);
>>>>>                esp_raise_drq(s);
>>>>>                return 0;
>>>>>            }
>>>>> @@ -316,24 +309,24 @@ static void satn_pdma_cb(ESPState *s)
>>>>>        if (get_cmd_cb(s) < 0) {
>>>>>            return;
>>>>>        }
>>>>> -    if (s->pdma_cur != s->pdma_start) {
>>>>> -        do_cmd(s, get_pdma_buf(s) + s->pdma_start);
>>>>> +    s->do_cmd = 0;
>>>>> +    if (s->cmdlen) {
>>>>> +        do_cmd(s, s->cmdbuf);
>>>>
>>>> I don't understant how you can remove the pdma_start: normally it is here to keep track of the
>>>> position in the pDMA if the migration is occuraing while a pDMA transfer.
>>>
>>> Hi Laurent,
>>>
>>> I was going to follow up on your reviews later this evening, however this one caught my eye: as per
>>> the cover letter, this patchset is a migration break for the q800 machine. As there are likely more
>>> incompatible changes for the q800 machine coming up soon, it didn't seem worth the effort (and
>>> indeed I don't think it's possible to recreate the new internal state with 100% accuracy from the
>>> old state).
>>>
>>> Migration for ESP devices that don't use PDMA is still supported, and I've tested this during
>>> development with qemu-system-sparc.
>>>
>>
>> I don't mean we can't migrate from a previous version to the new one, I mean the migration between
>> two machines of the current version is not possible anymore as we don't keep track of the position
>> of the pDMA index inside the buffer.
>>
>> With a DMA, the migration cannot happen in the middle of the DMA, while with pDMA it can (as it's a
>> processor loop).
>>
>> The whole purpose of get_pdma() and set_pdma() was for the migration.
>>
>> https://patchew.org/QEMU/20190910113323.17324-1-laurent@vivier.eu/diff/20190910193347.16000-1-laurent@vivier.eu/
>>
>>
>> Previously the Q800 was not migratable also because the CPU was not migratable, but I added recently
>> the VMSTATE for the CPU.
> 
> What should happen here is that the PDMA bytes for the message out and command phases are
> accumulated in cmdbuf and cmdlen as per normal ESP DMA - these are already included in the migration
> stream so there should be no problem there.
> 
> The PDMA callbacks are invoked when pdma_len == 0 where pdma_len is initially set to len in
> esp_do_dma: this is effectively the TC which is set to the length of the CDB for a DMA transfer.
> This means that the PDMA callback and hence do_cmd() are only called at the end of the transfer once
> the entire CDB has been accumulated where pdma_start is 0 (cmdbuf always includes the preceding
> IDENTIFY byte).
>

OK, I think I was worried about the migration of the value of pdma_cur, used in
sysbus_esp_pdma_read()/sysbus_esp_pdma_write(), but it seems you take care of that later in the
series...

Thanks,
Laurent