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[Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2
From: |
Peter Maydell |
Subject: |
[Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198 |
Date: |
Wed, 02 Dec 2020 17:36:18 -0000 |
Yes, we should by default do what the Linux kernel does, but we should
also provide a mechanism for allowing guest software to use a higher
vector length than that kernel default. On a real kernel you can do that
by either setting the /proc/sys/abi/sve_default_vector_length, or by
having process A make the prctl() to change vector length and then exec
process B that inherits that increased vector length. Neither of those
mechanisms work for QEMU linux-user, so we should provide some other
mechanism instead.
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https://bugs.launchpad.net/bugs/1906536
Title:
Unable to set SVE VL to 1024 bits or above since 7b6a2198
Status in QEMU:
New
Bug description:
Prior to 7b6a2198e71794c851f39ac7a92d39692c786820, the QEMU option
sve-max-vq could be used to set the vector length of the
implementation. This is useful (among other reasons) for testing
software compiled with a fixed SVE vector length. Since this commit,
the vector length is capped at 512 bits.
To reproduce the issue:
$ cat rdvl.s
.global _start
_start:
rdvl x0, #1
asr x0, x0, #4
mov x8, #93 // exit
svc #0
$ aarch64-linux-gnu-as -march=armv8.2-a+sve rdvl.s -o rdvl.o
$ aarch64-linux-gnu-ld rdvl.o
$ for vl in 1 2 4 8 16; do ../build-qemu/aarch64-linux-user/qemu-aarch64 -cpu
max,sve-max-vq=$vl a.out; echo $?; done
1
2
4
4
4
For a QEMU built prior to the above revision, we get the output:
1
2
4
8
16
as expected. It seems that either the old behavior should be restored,
or there should be an option to force a higher vector length?
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- [Bug 1906536] [NEW] Unable to set SVE VL to 1024 bits or above since 7b6a2198, Alex Coplan, 2020/12/02
- [Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198, Philippe Mathieu-Daudé, 2020/12/02
- [Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198, Alex Coplan, 2020/12/02
- [Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198, Peter Maydell, 2020/12/02
- [Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198,
Peter Maydell <=
- [Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198, Alex Bennée, 2020/12/03
- [Bug 1906536] Re: Unable to set SVE VL to 1024 bits or above since 7b6a2198, Peter Maydell, 2020/12/03