On Fri, Aug 18, 2023 at 01:56:54PM -0300, Jason Gunthorpe wrote: > On Mon, Aug 14, 2023 at 11:18:46PM -0700, Nicolin Chen wrote: > > On Fri, Aug 11, 2023 at 09:45:21AM -0700, Nicolin Chen wrote: > > > > > > But if stepping back a bit supporting an array-based non-native format > > > > could simplify the uAPI design and allows code sharing for array among > > > > vendor drivers. You can still keep the entry as native format then the > > > > only difference with future in-kernel fast path is just on walking an array > > > > vs. walking a ring. And VMM doesn't need to expose non-invalidate > > > > cmds to the kernel and then be skipped. > > > > > > Ah, so we might still design the uAPI to be ring based at this > > > moment, yet don't support a case CONS > 0 to leave that to an > > > upgrade in the future. > > > > > > I will try estimating a bit how complicated to implement the > > > ring, to see if we could just start with that. Otherwise, will > > > just start with an array. > > > > I drafted a uAPI structure for a ring-based SW queue. While I am > > trying an implementation, I'd like to collect some comments at the > > structure, to see if it overall makes sense. > > I don't think a ring makes alot of sense at this point. The only thing > it optimizes is a system call if the kernel needs to wrap around the > tail of the ring. It would possibly be better to have a small gather > list than try to describe the ring logic.. > > Further, the VMM already has to process it, so the vmm already knows > what ops are going to kernel are not. The VMM can just organize them > in a linear list in one way or another. We need to copy and read this > stuff in the VMM anyhow to protect against a hostile VM. OK. Then an linear array it is. > > One thing that I couldn't add to this common structure for SMMU > > is the hardware error code, which should be encoded in the higher > > bits of the consumer index register, following the SMMU spec: > > ERR, bits [30:24] Error reason code. > > - When a command execution error is detected, ERR is set to a > > reason code and then the SMMU_GERROR.CMDQ_ERR global error > > becomes active. > > - The value in this field is UNKNOWN when the CMDQ_ERR global > > error is not active. This field resets to an UNKNOWN value. > > The invalidate ioctl should fail in some deterministic way and report > back the error code and the highest array index that maybe could have > triggered it. Yea. Having an error code in the highest bits of array_index, "array_index != array_max" could be the deterministic way to indicate a failure. And a kernel errno could be returned also to the invalidate ioctl. > The highest array index sounds generic, the error code maybe is too We could do in its and report the error code in its raw form: __u32 out_array_index; /* number of bits used to report error code in the returned array_index */ __u32 out_array_index_error_code_bits; Or just: __u32 out_array_index; __u32 out_error_code; Do we have to define a list of generic error code? Thanks! Nic
