Re: [Qemu-devel] [Qemu-block] [RFC 1/1] ide: bug #1777315: io_buffer_size and sg.size can represent partial sector sizes

[Amol Surati]

On Tue, Jun 19, 2018 at 09:45:15AM -0400, John Snow wrote:
> 
> 
> On 06/19/2018 04:53 AM, Kevin Wolf wrote:
> > Am 19.06.2018 um 06:01 hat Amol Surati geschrieben:
> >> On Mon, Jun 18, 2018 at 08:14:10PM -0400, John Snow wrote:
> >>>
> >>>
> >>> On 06/18/2018 02:02 PM, Amol Surati wrote:
> >>>> On Mon, Jun 18, 2018 at 12:05:15AM +0530, Amol Surati wrote:
> >>>>> This patch fixes the assumption that io_buffer_size is always a perfect
> >>>>> multiple of the sector size. The assumption is the cause of the firing
> >>>>> of 'assert(n * 512 == s->sg.size);'.
> >>>>>
> >>>>> Signed-off-by: Amol Surati <address@hidden>
> >>>>> ---
> >>>>
> >>>> The repository https://github.com/asurati/1777315 contains a module for
> >>>> QEMU's 8086:7010 ATA controller, which exercises the code path
> >>>> described in [RFC 0/1] of this series.
> >>>>
> >>>
> >>> Thanks, this made it easier to see what was happening. I was able to
> >>> write an ide-test test case using this source as a guide, and reproduce
> >>> the error.
> >>>
> >>> static void test_bmdma_partial_sector_short_prdt(void)
> >>> {
> >>>     QPCIDevice *dev;
> >>>     QPCIBar bmdma_bar, ide_bar;
> >>>     uint8_t status;
> >>>
> >>>     /* Read 2 sectors but only give 1 sector in PRDT */
> >>>     PrdtEntry prdt[] = {
> >>>         {
> >>>             .addr = 0,
> >>>             .size = cpu_to_le32(0x200),
> >>>         },
> >>>         {
> >>>             .addr = 512,
> >>>             .size = cpu_to_le32(0x44 | PRDT_EOT),
> >>>         }
> >>>     };
> >>>
> >>>     dev = get_pci_device(&bmdma_bar, &ide_bar);
> >>>     status = send_dma_request(CMD_READ_DMA, 0, 2,
> >>>                               prdt, ARRAY_SIZE(prdt), NULL);
> >>>     g_assert_cmphex(status, ==, 0);
> >>>     assert_bit_clear(qpci_io_readb(dev, ide_bar, reg_status), DF | ERR);
> >>>     free_pci_device(dev);
> >>> }
> >>>
> >>>> Loading the module reproduces the bug. Tested on the latest master
> >>>> branch.
> >>>>
> >>>> Steps:
> >>>> - Install a Linux distribution as a guest, ensuring that the boot disk
> >>>>   resides on non-IDE controllers (such as virtio)
> >>>> - Attach another disk as a master device on the primary
> >>>>   IDE controller (i.e. attach at -hda.)
> >>>> - Blacklist ata_piix, pata_acpi and ata_generic modules, and reboot.
> >>>> - Copy the source files into the guest and build the module.
> >>>> - Load the module. QEMU process should die with the message:
> >>>>   qemu-system-x86_64: hw/ide/core.c:871: ide_dma_cb:
> >>>>   Assertion `n * 512 == s->sg.size' failed.
> >>>>
> >>>>
> >>>> -Amol
> >>>>
> >>>
> >>> I'm less sure of the fix -- certainly the assert is wrong, but just
> >>> incrementing 'n' is wrong too -- we didn't copy (n+1) sectors, we copied
> >>> (n) and a few extra bytes.
> >>
> >> That is true.
> >>
> >> There are (at least) two fields that represent the total size of a DMA
> >> transfer -
> >> (1) The size, as requested through the NSECTOR field.
> >> (2) The size, as calculated through the length fields of the PRD entries.
> >>
> >> It makes sense to consider the most restrictive of the sizes, as the factor
> >> which determines both the end of a successful DMA transfer and the
> >> condition to assert.
> >>
> >>>
> >>> The sector-based math here would need to be adjusted to be able to cope
> >>> with partial sector reads... or we ought to avoid doing any partial
> >>> sector transfers.
> >>>
> >>>
> >>> I'm not sure which is more correct tonight, it depends:
> >>>
> >>> - If it's OK to transfer partial sectors before reporting overflow,
> >>> adjusting the command loop to work with partial sectors is OK.
> >>>
> >>> - If it's NOT OK to do partial sector transfer, the sglist preparation
> >>> phase needs to produce a truncated SGList that's some multiple of 512
> >>> bytes that leaves the excess bytes in a second sglist that we don't
> >>> throw away and can use as a basis for building the next sglist. (Or the
> >>> DMA helpers need to take a max_bytes parameter and return an sglist
> >>> representing unused buffer space if the command underflowed.)
> >>
> >> Support for partial sector transfers is built into the DMA interface's PRD
> >> mechanism itself, because an entry is allowed to transfer in the units of
> >> even number of bytes.
> >>
> >> I think the controller's IO process runs in two parts (probably loops over
> >> for a single transfer):
> >>
> >> (1) The controller's disk interface transfers between its internal buffer
> >>     and the disk storage. The transfers are likely to be in the
> >>     multiples of a sector.
> >> (2) The controller's DMA interface transfers between its internal buffer
> >>     and the system memory. The transfers can be sub-sector in size(, and
> >>     are preserving of the areas, of the internal buffer, not subject to a
> >>     write.)
> > 
> > The spec isn't clear about this (or at least I can't find anything where
> > the exact behaviour is specified), but I agree that that's my mental
> > model as well. So I would make IDE send a byte-granularity request with
> > the final partial sector to the block layer, so that the data is
> > actually transferred up to that point.
> > 
> > In practice it probably doesn't matter much because a too short PRDT
> > means that the request doesn't complete successfully (the condition is
> > indicated by clear Interrupt, Active and Error flags in the BMDMA
> > controller) and I suppose the guest won't actually look at the data
> > then.
> > 
> > Providing the data anyway (consistent with our assumption how real
> > hardware works) is erring on the safe side because it doesn't hurt a
> > reasonable guest that did not expect the data to be transferred in this
> > condition.
> > 
> > Kevin
> > 
> 
> Partial transfers it is, since I didn't see anything in AHCI or BMDMA
> that suggested we shouldn't and it seems like the actual easiest fix
> because it avoids having to modify the sglists or the sglist preparation
> functions.
> 
> Amol, would you like to author a fix, or would you prefer that I do it?

Yes, I would like to author it. I assume that the simplification of the
calls to prepare_buf is better kept as a change that is separate from
this fix.

> 
> If you do, please copy my ide-test case above and check it in as patch
> 2/2 to your series as a regression test (tests/ide-test.c). It may need
> some further editing after the command submission to pass; I only made
> sure it crashes QEMU.

Will do.

Thanks,
-amol

> 
> Thanks,
> --js