Re: [PATCH 0/2] RISC-V: Probe for misaligned access speed

[Evan Green]

On Mon, Jun 26, 2023 at 2:24 AM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Yangyu Chen
> > Sent: 24 June 2023 11:22
> >
> > Hi,
> >
> > Thanks for doing this.
> >
> > On 6/24/23 6:20 AM, Evan Green wrote:
> > > I don't have a machine where misaligned accesses are slow, but I'd be
> > > interested to see the results of booting this series if someone did.
> >
> > I have tested your patches on a 100MHz BigCore rocket-chip with opensbi running on FPGA with
> > 72bit(64bit+ECC) DDR3 1600MHz memory. As the rocket-chip did not support misaligned memory access,
> > every misaligned memory access will trap and emulated by SBI.
> >
> > Here is the result:
> ...
> > ~ # dmesg | grep Unaligned
> > [    0.210140] cpu1: Unaligned word copy 0 MB/s, byte copy 38 MB/s, misaligned accesses are slow
> > [    0.410715] cpu0: Unaligned word copy 0 MB/s, byte copy 35 MB/s, misaligned accesses are slow
>
> How many misaligned cycles are in the test loop?
> If emulated ones are that slow you pretty much only need to test one.

The code does as many cycles as it can in a fixed number of jiffies.

>
> Also it is pretty clear that you really don't want to be emulating them.
> If the emulation is hidden from the kernel that really doesn't help at all.

>From what I understand there's work being done to give the kernel some
awareness and even control over the misaligned access
trapping/emulation. It won't help today's systems though, and either
way you're right emulating is very very slow.
-Evan