Re: [RISC-V] [tech-cmo] Fault-on-first should be allowed to return randomly on non-faults (also, running SIMT code on vector ISA)


David Horner
 

First I am very happy that "arbitrary decisions by the micro-architecture" allow reduction of vl to any [non-zero] value.

Even if such appear "random".


On 2020-10-16 2:01 a.m., krste@berkeley.edu wrote:
- I'm sure there's probably
papers out there with this already).
Exactly.
I see this openness/lack of arbitrary constraint  as  precisely  the strength of RISCV.
Limiting vector operations due to current constraints in software (Linuz does it this way, compilers cannot optimize that formulation[yet])
or hardware (reminiscent of delayed branch because prediction was too expensive) is short sighted.

A check for vl=0 on platforms that allow it is eminently doable, low overhead for many use cases  AND guarantees forward progress under SOFTWARE control.

I see it as no different [in fundamental principle] than other cases such as RVI integer divide by zero behaviour that does not trap but can be  readily checked for.
Also RVI integer overflow that if you want to check for it is at most a few instructions including the branch.
(sending replies to vector list - as this is off topic for CMOs)

My opinion is that baking SIMT execution model into ISA for purposes
of exposing microarchitectural performance (i.e., cache misses)
exposes too much of the machine, forcing application software to add
extra retry loops (2nd nested loop inside of stripmining) and forcing
system software to deal with complex traps.

[ Random historical connection - having a partial completion mask based
on cache misses is a vector version of the Stanford proposal for
"informing memory operations" where scalar core can branch on cache miss.
https://dl.acm.org/doi/10.1145/232974.233000 ]
Most of the benefit for SIMT execution around microarchitectural
hiccups can be obtained under the hood in the microarchitecture (and
there are several hundred ISCA/MICRO/HPCA papers on doing that - I
might be exaggerating, but only slightly - and I know Andy worked in
this space at some point), and should outperform putting this handling
into software.

That said, I think it's OK to allow FF V loads to stop anywhere past
element 0 including at a long-latency cache miss, mainly because it
doesn't change anything in software model.

I'm not sure it will really help perf that much in practice. While
it's easy to construct an example where it looks like it would help, I
think in general most loops touch multiple vector operands, hardware
prefetchers do well on vector streams, vector units are more efficient
on larger chunks, scatter-gathers missing in cache limit perf anyway,
etc., so it's probably a fairly brittle optimization (yes, you could
add a predictor to figure out whether to wait for the other elements
or go ahead with a partial vector result - I'm sure there's probably
papers out there with this already).

Krste

On Fri, 16 Oct 2020 04:03:17 +0000, "Bill Huffman" <huffman@cadence.com> said:
| My take is the same as Andrew has outlined below.
| Bill

| On 10/15/20 4:30 PM, andrew@sifive.com wrote:

| EXTERNAL MAIL
| Forwarding this to tech-vector-ext; couple comments below.
| On Thu, Oct 15, 2020 at 2:33 PM Andy Glew Si5 <andy.glew@sifive.com> wrote:

| In vector meeting last Friday I listened to both Krste and David Horner's different opinions about fault-on-first and vector length trimming. I realized (and may have
| convinced other attendees) that the RISC-V "fault-on-first" vector length trimming need not be done just for things like page-faults.
| Fault-on-first could be done for the first long latency cache miss, as long as vector element zero has been completed, because vector element zero is the forward progress
| mechanism.
| Indeed, IMHO the correct semantic requirement for fault-on-first is that it completes the element zero of the operation, but that it can randomly stop with the appropriate
| indication for vector length trimming at any point in the middle of the instruction.
| Indeed, I've found other microarchitectural reasons to favor this approach (e.g., speculating through mask-register values). Enumerating all cases in which the length might be
| trimmed seems like a fool's errand, so just saying it can be truncated to >= 1 for any reason is the way to go.

| This is part of what David Horner wants. However, it does not give him the fault-on-first with zero length complete mechanism. It could, if there were something else in
| the system that guaranteed forward progress
| My take is that requiring that element 0 either complete or trap is already a solid mechanism for guaranteeing forward progress, and cleanly matches the while-loop vectorization
| model.

| ---+ Expanded

| From vector meeting last Friday: trimming, fault-on-first. I realized that it is similar to the forms of SW visible non-faulting speculative loads some machines, especially
| VLIWs, have. However, instead of delivering a NaN or NaT, it is non-faulting except for vector element 0, where it faults. The NaT-ness is implied by trimmed vector length.
| It could be implied by a mask showing which vector operations had completed.

| All such SW non-faulting loads need a "was this correct" operation, which might just be a faulting load and a comparison. Software control flow must fall through such a
| check operation, and through a redo of the faulting load if necessary. In scalar, non-faulting and faulting loads are different instructions, so there must be a branch.

| The RISC-V Fault-on-first approach has the correctness check for non-faulting implied by redoing the instruction. i.e. it is its own non-faulting check. it gets away with
| this because the trend vector length indicates which parts were valid and not. forward progress is guaranteed by trapping on vector element zero, i.e. never allowing a trim
| to zero length. if a non-faulting vector approach was used instead of fault-on-first, it could return a vector complete mask, but to make forward progress it would have to
| guarantee that at least one vector element had completed.

| David Horner's desire for fault-on-first that may have performed no operations at all is (1) reasonable IMHO (I think I managed to explain that the Krste), but (2) Would
| require some other mechanism for forward progress. E.g. instead of trapping on element zero, the bitmask that I described above. Which is almost certainly a bigger
| architectural change than RISC-V should make it this time.

| Although more and more I am happier that I included such a completion bitmask in newly every vector instruction set that I've ever done. Particularly those vector instruction
| sets that were supposed to implement SIMT efficiently. (I think of SIMT as a programming model that is implemented on top of what amounts to a vector instruction set and
| microarchitecture. https://pharr.org/matt/papers/ispc_inpar_2012.pdf ). It would be unfortunate for such an SIMT program to lose work completed after the first fault.

| MORAL: fault-on-first may be suitable for vector load that might speculate past the end of the vector - where the length is not known or inconvenient when the vector load
| instruction is started. Fault-on-first is suboptimal for running SIMT on top of vectors. i.e. fault-on-first is the equivalent of precise exceptions for in order
| execution, and for a single thread executing vector instructions, whereas completion mask allows out of order within a vector and/or vector length threading.

| IMHO an important realization I made in that meeting is that fault-on-first does not need to be just about faulting. It is totally fine to have the fault-on-first stuff
| return up to the first really long latency cost miss, as long as it always guarantees that at least vector element zero was complete. Because vector element zero complete
| is what guarantees forward progress.

| Furthermore, it is not even required that fault-on-first stop at the first page-fault. An implementation could actually choose to actually implement a page-fault that did
| copy-on-write or swapped in from disk. but that would be visible to the operating system, not the user program. However, such an OS implementation would have to
| guarantee that it would not kill a process as a result of a true permissions error page-fault. Or, if the virtual memory architecture made the distinction between
| permissions faults and the sorts of page-fault that is for disk swapping or copy-on-write or copy on read, the OS does not need to be involved.

| EVERYTHING about fault-on-first is a microarchitecture security/information leak channel and/or a virtualization hole. (Unless you only trim only on true faults and not COW
| or COR or disk swappage-faults). However, fault-on-first on any page-fault is a much lower bandwidth information leak channel than is fault-on-first on long latency
| cache misses. so a general purpose system might choose to implement fault-on-first on any page-fault, but might not want to implement fault-on-first on any cache miss.
| However, there are some systems for which that sort of security issue is not a concern. E.g. a data center or embedded system where all of the CPUs are dedicated to a single
| problem. In which case, if they can gain performance by doing fault-on-first on particular long latency cache misses, power to them!

| Interestingly, although fault-on-first on long latency cache misses is a high-bandwidth information leak, it is actually much less of a virtualization hole than
| fault-on-first for page-faults. The operating system or hypervisor has very little control over cache misses. the OS and hypervisor have almost full control over
| page-faults. The usual rule in security and virtualization is that an application should not be able to detect that it has had an "innocent" page-fault, such as COW or COR
| or disk swapping.

| --
| --- Sorry: Typos (Speech-Os?) Writing Errors <= Speech Recognition <= Computeritis
|

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