Messages in this thread Patch in this message | | | Date | Tue, 7 Mar 2023 11:12:00 +0000 | From | Jonathan Cameron <> | Subject | Re: [PATCH] cxl/hdm: Fix hdm decoder init by adding COMMIT field check |
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On Mon, 6 Mar 2023 16:04:22 +0000 Jonathan Cameron <Jonathan.Cameron@Huawei.com> wrote:
> On Fri, 3 Mar 2023 17:21:13 +0000 > Fan Ni <fan.ni@samsung.com> wrote: > > > On Fri, Mar 03, 2023 at 02:36:05PM +0000, Jonathan Cameron wrote: > > > > > On Thu, 2 Mar 2023 08:36:59 -0700 > > > Dave Jiang <dave.jiang@intel.com> wrote: > > > > > > > On 3/1/23 11:23 PM, Fan Ni wrote: > > > > > On Wed, Mar 01, 2023 at 11:54:08AM -0700, Dave Jiang wrote: > > > > >> > > > > > Hi Dave, > > > > > Thanks for looking into this. > > > > >> > > > > >> On 2/28/23 3:40 PM, Fan Ni wrote: > > > > >>> Add COMMIT field check aside with existing COMMITTED field check during > > > > >>> hdm decoder initialization to avoid a system crash during module removal > > > > >>> after destroying a region which leaves the COMMIT field being reset while > > > > >>> the COMMITTED field still being set. > > > > >> > > > > >> Hi Fan. Are you seeing this issue on qemu emulation or hardware? The > > > > > I run into the issue with qemu emulation. > > > > >> situation does not make sense to me. If we clear the COMMIT bit, then the > > > > >> COMMITTED bit should be cleared by the hardware shortly after right? > > > > > > > > > > From the spec, I cannot find any statement saying clearing the COMMIT bit > > > > > will automatically clear the COMMITTED. If I have not missed the statement in > > > > > the spec, I assume we should not make the assumption that it will be > > > > > cleared automatically for real hardware. But you may be right, leaving the > > > > > COMMITTED bit set can potentially cause some issue? Need to check more. > > > > > > > > I have not been able to find direct verbiage that indicates this either. > > > > However, logically it would make sense. Otherwise, the COMMITTED field > > > > never clears and prevents reprogramming of the HDM decoders. The current > > > > QEMU implementation is creating a situation where the HDM decoder is > > > > always active after COMMIT bit is set the first time, regardless whether > > > > COMMIT field has been cleared later on during a teardown. It does sound > > > > like a bug with QEMU emulation currently. > > > > > > I agree that one sane interpretation is that unsetting commit should result in > > > the decoder being deactivated and hence the commit bit dropping. However > > > I'm not sure that's the only sane interpretation. > > > > > > There is no verbage that I'm aware of that says the committed bit being > > > set means that the current register values are in use. It simply says that > > > when the commit bit was set, the HDM decoder was successfully committed > > > (using registers as set at that time). There is a specific statement about > > > not changing the registers whilst checks are in progress, but those checks > > > are only required if lock on commit is set, so it doesn't cover this case. > > > > > > Wonderfully there isn't actually anything says what a commit transition to 0 > > > means. Does that result in the decoder become uncommitted, or does that only > > > happen when the next 0 to 1 transition happens? > > > > > > The only stuff we have is what happens when lock on commit = 1, which isn't > > > the case here. > > > > > > So is there another valid implementation? I think yes. > > > In some implementations, there will be a complex state machine that is > > > triggered when commit is set. That will then write some entirely invisible > > > internal state for decode logic based on the contents of the registers. > > > As such, once it's set committed, it typically won't look at the registers > > > again until another commit 0->1 transition happens. At that point the > > > committed bit drops and raised again once the commit state machine finishes > > > (given QEMU doesn't emulate that delay the upshot is if you set commit then > > > check committed it will be set ;) > > > > > > In that implementation the commit 1->0 transition is an irrelevance and > > > it won't change the committed bit state. > > > > > > So whilst the QEMU code is doing the less obvious implementation, I think > > > the spec still allows it. I don't mind QEMU changing to the more obvious > > > one though if someone wants to send a patch. > > > > > > Jonathan > > > > > > > In current qemu emulation, when COMMITTED bit is set when the decoder is > > committed and at the same time the COMMIT field will be cleared. Does > > the following fix make sense? > > 1. At qemu side, when the commit completes, just set the COMMITTED bit, > > but leave the COMMIT bit as set, also check LOCK ON COMMIT bit, > > if it is set, clear it, which will allow further reset of COMMIT bit. > > QEMU definitely can't do anything to the Commit bit, other than prevent it being > cleared if lock on commit is set. > Right now the QEMU emulation doesn't handle LOCK ON COMMIT at all. > It would be sensible to add this support, but we don't have an > open software stack that ever sets that yet so any testing is likely to be > one time only via some hacks. > > > 2. for the kernel side, if it needs to reprogram the decoder, it needs to > > check the COMMITTED bit, if it is set, then OS need to reset COMMIT bit > > first, which will also clear COMMITTED bit automatically at qemu side. > > Could do it that way, or simplify it by always clearing commit before setting > it to make sure the transition happens. > > Looks like commit is cleared in cxl_decoder_reset() already so this may > already happen - I haven't checked the flow. > > > 3. when the OS needs to reset the decoder, it does similar thing as 2 to > > reset COMMIT bit and qemu will clear COMMITTED bit. > > No the point of the above argument is that the spec doesn't say anything > about when committed is cleared. 2 options. > 1) Hardware clears it when commit 1->0. > 2) Hardware clears it when commit 0->1 > > Given that spec only talks about after a commit 0->1 transition whilst commit > remains 1, the state after a commit 0->1 transition is implementation defined. > > I think that closing that corner case requires a clarification to the spec. > > Which leaves us with a sticky question of what to do...
Thinking a little more on this and another close look at spec. The committed bit definition calls out "Indicates a decoder is active" so if it is not cleared when commit 1->0 then we may have a problem with multiple decoders and the clear only on commit 0->1 option
Let us first setup decoders as. decoder 0 -> HPA X to X + N1 (then commit) decoder 1 -> HPA X + N1 to X + N1 + M1 (then commit)
Now we want to change them without passing through a situation where we have overlap so that we have N2 > N1. There is a route to doing this but it's not very intuitive.
1. Unset commit on both decoders 2. Update decoder 1 first and commit. Have to do it in this order as decoder 0 is still committed (in use) so we can't overlap with it. 3. Update decoder 0 second and commit.
If N1 < N2 need to reverse the order.
1. Unset commit on both decoders 2. Update decoder 0 first and commit. Avoids overlap with still committed decoder 1. 3. Update decoder 1 and commit.
So I think there is a path to make it work but it's nasty.
I'll raise a query with CXL SSWG chair (off list but referring to this thread)
Jonathan
> > > > > Fan > > > > > > > > > > DJ > > > > > > > > > > > > > > Fan > > > > > > > > > >> Otherwise, how would one reprogram the decoder if the decoder is indicating > > > > >> to be active? > > > > >> > > > > >> DJ > > > > >> > > > > >>> > > > > >>> In current kernel implementation, when destroying a region (cxl > > > > >>> destroy-region),the decoders associated to the region will be reset > > > > >>> as that in cxl_decoder_reset, where the COMMIT field will be reset. > > > > >>> However, resetting COMMIT field will not automatically reset the > > > > >>> COMMITTED field, causing a situation where COMMIT is reset (0) while > > > > >>> COMMITTED is set (1) after the region is destroyed. Later, when > > > > >>> init_hdm_decoder is called (during modprobe), current code only check > > > > >>> the COMMITTED to decide whether the decoder is enabled or not. Since > > > > >>> the COMMITTED will be 1 and the code treats the decoder as enabled, > > > > >>> which will cause unexpected behaviour. > > > > >>> > > > > >>> Before the fix, a system crash was observed when performing following > > > > >>> steps: > > > > >>> 1. modprobe -a cxl_acpi cxl_core cxl_pci cxl_port cxl_mem > > > > >>> 2. cxl create-region -m -d decoder0.0 -w 1 mem0 -s 256M > > > > >>> 3. cxl destroy-region region0 -f > > > > >>> 4. rmmod cxl_acpi cxl_pci cxl_port cxl_mem cxl_pmem cxl_core > > > > >>> 5. modprobe -a cxl_acpi cxl_core cxl_pci cxl_port cxl_mem (showing > > > > >>> "no CXL window for range 0x0:0xffffffffffffffff" error message) > > > > >>> 6. rmmod cxl_acpi cxl_pci cxl_port cxl_mem cxl_pmem cxl_core (kernel > > > > >>> crash at cxl_dpa_release due to dpa_res has been freed when destroying > > > > >>> the region). > > > > >>> > > > > >>> The patch fixed the above issue, and is tested based on follow patch series: > > > > >>> > > > > >>> [PATCH 00/18] CXL RAM and the 'Soft Reserved' => 'System RAM' default > > > > >>> Message-ID: 167601992097.1924368.18291887895351917895.stgit@dwillia2-xfh.jf.intel.com > > > > >>> > > > > >>> Signed-off-by: Fan Ni <fan.ni@samsung.com> > > > > >>> --- > > > > >>> drivers/cxl/core/hdm.c | 8 +++++--- > > > > >>> 1 file changed, 5 insertions(+), 3 deletions(-) > > > > >>> > > > > >>> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c > > > > >>> index 80eccae6ba9e..6cf854c949f0 100644 > > > > >>> --- a/drivers/cxl/core/hdm.c > > > > >>> +++ b/drivers/cxl/core/hdm.c > > > > >>> @@ -695,6 +695,7 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld, > > > > >>> struct cxl_endpoint_decoder *cxled = NULL; > > > > >>> u64 size, base, skip, dpa_size; > > > > >>> bool committed; > > > > >>> + bool should_commit; > > > > >>> u32 remainder; > > > > >>> int i, rc; > > > > >>> u32 ctrl; > > > > >>> @@ -710,10 +711,11 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld, > > > > >>> base = ioread64_hi_lo(hdm + CXL_HDM_DECODER0_BASE_LOW_OFFSET(which)); > > > > >>> size = ioread64_hi_lo(hdm + CXL_HDM_DECODER0_SIZE_LOW_OFFSET(which)); > > > > >>> committed = !!(ctrl & CXL_HDM_DECODER0_CTRL_COMMITTED); > > > > >>> + should_commit = !!(ctrl & CXL_HDM_DECODER0_CTRL_COMMIT); > > > > >>> cxld->commit = cxl_decoder_commit; > > > > >>> cxld->reset = cxl_decoder_reset; > > > > >>> - if (!committed) > > > > >>> + if (!should_commit || !committed) > > > > >>> size = 0; > > > > >>> if (base == U64_MAX || size == U64_MAX) { > > > > >>> dev_warn(&port->dev, "decoder%d.%d: Invalid resource range\n", > > > > >>> @@ -727,7 +729,7 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld, > > > > >>> }; > > > > >>> /* decoders are enabled if committed */ > > > > >>> - if (committed) { > > > > >>> + if (should_commit && committed) { > > > > >>> cxld->flags |= CXL_DECODER_F_ENABLE; > > > > >>> if (ctrl & CXL_HDM_DECODER0_CTRL_LOCK) > > > > >>> cxld->flags |= CXL_DECODER_F_LOCK; > > > > >>> @@ -772,7 +774,7 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld, > > > > >>> return 0; > > > > >>> } > > > > >>> - if (!committed) > > > > >>> + if (!should_commit || !committed) > > > > >>> return 0; > > > > >>> dpa_size = div_u64_rem(size, cxld->interleave_ways, &remainder); > > > > >
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