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Re: [Qemu-devel] [PATCH v5 2/5] hw/block/pflash_cfi01: Use the correct R
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v5 2/5] hw/block/pflash_cfi01: Use the correct READ_ARRAY value |
Date: |
Tue, 16 Jul 2019 15:04:43 +0100 |
User-agent: |
Mutt/1.12.0 (2019-05-25) |
* Markus Armbruster (address@hidden) wrote:
> Philippe asked me to have a look at this one, so here goes.
>
> Philippe Mathieu-Daudé <address@hidden> writes:
>
> > In the document [*] the "Read Array Flowchart", the READ_ARRAY
> > command has a value of 0xff.
> >
> > Use the correct value in the pflash model.
> >
> > There is no change of behavior in the guest, because:
> > - when the guest were sending 0xFF, the reset_flash label
> > was setting the command value as 0x00
> > - 0x00 was used internally for READ_ARRAY
>
> *Groan*
>
> Is this cleanup, or does it fix an observable bug?
>
> > To keep migration with older versions behaving correctly, we
> > decide to always migrate the READ_ARRAY as 0x00.
> >
> > If the CFI open standard decide to assign a new command of value
> > 0x00, this model is flawed because it uses this value internally.
> > If a guest eventually requires this new CFI feature, a different
> > model will be required (or this same model but breaking backward
> > migration). So it is safe to keep migrating READ_ARRAY as 0x00.
>
> We could perhaps keep migration working for "benign" device states, with
> judicious use of subsections. We'll cross that bridge when we get to
> it.
>
> > [*] "Common Flash Interface (CFI) and Command Sets"
> > (Intel Application Note 646)
> > Appendix B "Basic Command Set"
> >
> > Reviewed-by: John Snow <address@hidden>
> > Reviewed-by: Alistair Francis <address@hidden>
> > Regression-tested-by: Laszlo Ersek <address@hidden>
> > Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> > ---
> > v3: Handle migrating the 'cmd' field.
> > v4: Handle migrating to older QEMU (Dave)
> > v5: Add a paragraph about why this model is flawed due to
> > historically using READ_ARRAY as 0x00 (Dave, Peter).
> >
> > Since Laszlo stated he did not test migration [*], I'm keeping his
> > test tag, because the change with v2 has no impact in the tests
> > he ran.
> >
> > Likewise I'm keeping John and Alistair tags, but I'd like an extra
> > review for the migration change, thanks!
> >
> > [*] https://lists.gnu.org/archive/html/qemu-devel/2019-07/msg00679.html
> > ---
> > hw/block/pflash_cfi01.c | 57 ++++++++++++++++++++++++++++++++++-------
> > 1 file changed, 48 insertions(+), 9 deletions(-)
> >
> > diff --git a/hw/block/pflash_cfi01.c b/hw/block/pflash_cfi01.c
> > index 9e34fd4e82..85bb2132c0 100644
> > --- a/hw/block/pflash_cfi01.c
> > +++ b/hw/block/pflash_cfi01.c
> > @@ -96,6 +96,37 @@ struct PFlashCFI01 {
> > bool old_multiple_chip_handling;
> > };
> >
> > +static int pflash_pre_save(void *opaque)
> > +{
> > + PFlashCFI01 *s = opaque;
> > +
> > + /*
> > + * Previous to QEMU v4.1 an incorrect value of 0x00 was used for the
> > + * READ_ARRAY command. To preserve migrating to these older version,
> > + * always migrate the READ_ARRAY command as 0x00.
> > + */
> > + if (s->cmd == 0xff) {
> > + s->cmd = 0x00;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > +static int pflash_post_save(void *opaque)
> > +{
> > + PFlashCFI01 *s = opaque;
> > +
> > + /*
> > + * If migration failed, the guest will continue to run.
> > + * Restore the correct READ_ARRAY value.
> > + */
> > + if (s->cmd == 0x00) {
> > + s->cmd = 0xff;
> > + }
> > +
> > + return 0;
> > +}
>
> Uh, this gives me a queasy feeling. Perhaps David can assuage it.
See the previous 4 versions of discussion....
> I figure the intent is to migrate PFlashCFI01 member @cmd value 0xFF as
> 0x00, for migration compatibility to and from older versions.
>
> You do this by monkey-patching it to 0x00 before migration, and to 0xFF
> afterwards. On the incoming side, you replace 0x00 by 0xFF, in
> pflash_post_load() below.
>
> Questions:
>
> * Can anything but the code that sends @cmd see the temporary 0x00 value
> between pflash_pre_save() and pflash_post_save()
It is the same pflash data structure; but all CPUs are stopped and we're
just walking the list of devices serialising them; so no nothing should
be seeing that value.
(There is another way to do this, which is to produce a temporary
structure at this point, populate the temporary structure and migrate
that)
Dave
> * Consider the matrix source \in { old, new } x dest \in { old, new } x
> @cmd on source in { 0x00, 0xFF }. What does migration put into @cmd
> on dest? Eight cases:
>
> source @cmd -> wire -> dest @cmd
> old 0x00 -> 0x00 -> old 0x00 (1)
> new 0xFF (2)
> old 0xFF -> 0xFF -> old 0xFF (3)
> new 0xFF (4)
> new 0x00 -> 0x00 -> old 0x00 (5)
> new 0xFF (6)
> new 0xFF -> 0x00 -> old 0x00 (7)
> new 0xFF (8)
>
> Old -> old (cases 1 and 3) is unaffected by this patch.
>
> New -> new leaves 0xFF unchanged (8). It changes 0x00 to 0xFF (6).
> Uh-oh. Can this happen? Rephrasing the question: can @cmd ever be
> 0x00 with this patch applied?
>
> Old -> new leaves 0xFF unchanged (4). It changes 0x00 to 0xFF (2),
> which I think is intentional.
>
> New -> old leaves 0x00 unchanged (5). It changes 0xFF to 0x00 (7),
> which I think is intentional.
>
> Old -> new -> old leaves 0x00 unchanged. Good. It changes 0xFF to
> 0x00. Uh-oh. Can @cmd ever be 0xFF before this patch?
>
> New -> old -> new leaves 0xFF unchanged. Good. It changes 0x00 to
> 0xFF. Same uh-oh as for new -> new.
>
> > +
> > static int pflash_post_load(void *opaque, int version_id);
> >
> > static const VMStateDescription vmstate_pflash = {
> > @@ -103,6 +134,8 @@ static const VMStateDescription vmstate_pflash = {
> > .version_id = 1,
> > .minimum_version_id = 1,
> > .post_load = pflash_post_load,
> > + .pre_save = pflash_pre_save,
> > + .post_save = pflash_post_save,
> > .fields = (VMStateField[]) {
> > VMSTATE_UINT8(wcycle, PFlashCFI01),
> > VMSTATE_UINT8(cmd, PFlashCFI01),
> > @@ -277,10 +310,9 @@ static uint32_t pflash_read(PFlashCFI01 *pfl, hwaddr
> > offset,
> > /* This should never happen : reset state & treat it as a read */
> > DPRINTF("%s: unknown command state: %x\n", __func__, pfl->cmd);
> > pfl->wcycle = 0;
> > - pfl->cmd = 0;
> > + pfl->cmd = 0xff;
> > /* fall through to read code */
> > - case 0x00:
> > - /* Flash area read */
> > + case 0xff: /* Read Array */
> > ret = pflash_data_read(pfl, offset, width, be);
> > break;
>
> On 0xFF, we no longer zap pfl->wcycle and pfl->cmd.
>
> On 0x00, we do.
>
> We zap pfl->cmd to 0xFF instead of 0x00. Same below after label
> error_flash and reset_flash. Related: initialization to 0xFF instead of
> 0x00 in pflash_cfi01_realize(). I *guess* these changes together ensure
> pfl->cmd can't become 0x00. Correct?
>
> > case 0x10: /* Single byte program */
> > @@ -448,8 +480,6 @@ static void pflash_write(PFlashCFI01 *pfl, hwaddr
> > offset,
> > case 0:
> > /* read mode */
> > switch (cmd) {
> > - case 0x00: /* ??? */
> > - goto reset_flash;
>
> On 0x00, we now use default: goto error_flash. Can this happen?
>
> > case 0x10: /* Single Byte Program */
> > case 0x40: /* Single Byte Program */
> > DPRINTF("%s: Single Byte Program\n", __func__);
> > @@ -526,7 +556,7 @@ static void pflash_write(PFlashCFI01 *pfl, hwaddr
> > offset,
> > if (cmd == 0xd0) { /* confirm */
> > pfl->wcycle = 0;
> > pfl->status |= 0x80;
> > - } else if (cmd == 0xff) { /* read array mode */
> > + } else if (cmd == 0xff) { /* Read Array */
> > goto reset_flash;
> > } else
> > goto error_flash;
> > @@ -553,7 +583,7 @@ static void pflash_write(PFlashCFI01 *pfl, hwaddr
> > offset,
> > } else if (cmd == 0x01) {
> > pfl->wcycle = 0;
> > pfl->status |= 0x80;
> > - } else if (cmd == 0xff) {
> > + } else if (cmd == 0xff) { /* read array mode */
>
> Your new comment is phrased the way you corrected in the previous hunk.
> Intentional?
>
> > goto reset_flash;
> > } else {
> > DPRINTF("%s: Unknown (un)locking command\n", __func__);
> > @@ -645,7 +675,7 @@ static void pflash_write(PFlashCFI01 *pfl, hwaddr
> > offset,
> error_flash:
> qemu_log_mask(LOG_UNIMP, "%s: Unimplemented flash cmd sequence "
> "(offset " TARGET_FMT_plx ", wcycle 0x%x cmd 0x%x value
> 0x%x)"
> "\n", __func__, offset, pfl->wcycle, pfl->cmd, value);
>
> reset_flash:
> > trace_pflash_reset();
> > memory_region_rom_device_set_romd(&pfl->mem, true);
> > pfl->wcycle = 0;
> > - pfl->cmd = 0;
> > + pfl->cmd = 0xff;
> > }
> >
> >
> > @@ -761,7 +791,7 @@ static void pflash_cfi01_realize(DeviceState *dev,
> > Error **errp)
> > }
> >
> > pfl->wcycle = 0;
> > - pfl->cmd = 0;
> > + pfl->cmd = 0xff;
> > pfl->status = 0;
> > /* Hardcoded CFI table */
> > /* Standard "QRY" string */
> > @@ -1001,5 +1031,14 @@ static int pflash_post_load(void *opaque, int
> > version_id)
> > pfl->vmstate = qemu_add_vm_change_state_handler(postload_update_cb,
> > pfl);
> > }
> > +
> > + /*
> > + * Previous to QEMU v4.1 an incorrect value of 0x00 was used for the
> > + * READ_ARRAY command.
> > + */
> > + if (pfl->cmd == 0x00) {
> > + pfl->cmd = 0xff;
> > + }
> > +
> > return 0;
> > }
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
[Qemu-devel] [PATCH v5 3/5] hw/block/pflash_cfi01: Extract pflash_mode_read_array(), Philippe Mathieu-Daudé, 2019/07/15
[Qemu-devel] [PATCH v5 4/5] hw/block/pflash_cfi01: Start state machine as READY to accept commands, Philippe Mathieu-Daudé, 2019/07/15
[Qemu-devel] [PATCH v5 5/5] hw/block/pflash_cfi01: Add the DeviceReset() handler, Philippe Mathieu-Daudé, 2019/07/15