On Tue, Jun 19, 2018 at 08:04:03PM +0530, Amol Surati wrote: > 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 <suratia...@gmail.com> > > >>>>> --- > > >>>> > > >>>> 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
After ide_dma_cb checks the return value from prepare_buf, it comments thus: "The PRDs were too short. Reset the Active bit, but don't raise an interrupt." So, QEMU already has a policy in place for short PRDs. I apologize, that I did not notice it earlier. It is enforced by the condition: "(s->bus->dma->ops->prepare_buf(s->bus->dma, s->io_buffer_size) < 512)". The definition of short PRDs it uses is: those which result in a "< 512" bytes transfer. Is there a reason to not define them, as those which result in a "< n * 512" bytes transfer, to begin with (or now)? (n = s->nsector). If the prepare_buf interface implementation and use, that comment, and the behaviour (drop and goto eot) which that comment points towards, are all consistent with one another, then, the condition "< 512" looks like a bug in itself, solving of which also solves this crash (hopefully, without making any other change.) Thanks, -amol
