* [PATCH] tcg/arm: Reduce vector alignment requirement for NEON
@ 2021-09-12 17:49 Richard Henderson
2021-09-13 8:07 ` Richard W.M. Jones
2021-09-13 11:07 ` Richard W.M. Jones
0 siblings, 2 replies; 6+ messages in thread
From: Richard Henderson @ 2021-09-12 17:49 UTC (permalink / raw)
To: qemu-devel; +Cc: peter.maydell, Richard W . M . Jones
With arm32, the ABI gives us 8-byte alignment for the stack.
While it's possible to realign the stack to provide 16-byte alignment,
it's far easier to simply not encode 16-byte alignment in the
VLD1 and VST1 instructions that we emit.
Remove the assertion in temp_allocate_frame, limit natural alignment
to the provided stack alignment, and add a comment.
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
I haven't seen the assertion with the various arm kernels that I happen
to have laying about. I have not taken the time to build the combo
from the bug report:
[ 0.000000] Linux version 5.14.0-60.fc36.armv7hl (mockbuild@buildvm-a32-12.iad2.fedoraproject.org) (gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1), GNU ld version 2.37-9.fc36) #1 SMP Mon Aug 30 14:08:34 UTC 2021
I thought about parameterizing this patch further, but I can't think of
another ISA that would be affected. (i686 clumsily changed its abi 20
years ago to avoid faulting on vector spills; other isas so far have
allowed vectors to be unaligned.)
r~
---
tcg/tcg.c | 8 +++++++-
tcg/arm/tcg-target.c.inc | 13 +++++++++----
2 files changed, 16 insertions(+), 5 deletions(-)
diff --git a/tcg/tcg.c b/tcg/tcg.c
index 4142d42d77..ca5bcc4635 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -3060,7 +3060,13 @@ static void temp_allocate_frame(TCGContext *s, TCGTemp *ts)
g_assert_not_reached();
}
- assert(align <= TCG_TARGET_STACK_ALIGN);
+ /*
+ * Assume the stack is sufficiently aligned.
+ * This affects e.g. ARM NEON, where we have 8 byte stack alignment
+ * and do not require 16 byte vector alignment. This seems slightly
+ * easier than fully parameterizing the above switch statement.
+ */
+ align = MIN(TCG_TARGET_STACK_ALIGN, align);
off = ROUND_UP(s->current_frame_offset, align);
/* If we've exhausted the stack frame, restart with a smaller TB. */
diff --git a/tcg/arm/tcg-target.c.inc b/tcg/arm/tcg-target.c.inc
index e5b4f86841..8515717435 100644
--- a/tcg/arm/tcg-target.c.inc
+++ b/tcg/arm/tcg-target.c.inc
@@ -2477,8 +2477,13 @@ static void tcg_out_ld(TCGContext *s, TCGType type, TCGReg arg,
tcg_out_vldst(s, INSN_VLD1 | 0x7d0, arg, arg1, arg2);
return;
case TCG_TYPE_V128:
- /* regs 2; size 8; align 16 */
- tcg_out_vldst(s, INSN_VLD1 | 0xae0, arg, arg1, arg2);
+ /*
+ * We have only 8-byte alignment for the stack per the ABI.
+ * Rather than dynamically re-align the stack, it's easier
+ * to simply not request alignment beyond that. So:
+ * regs 2; size 8; align 8
+ */
+ tcg_out_vldst(s, INSN_VLD1 | 0xad0, arg, arg1, arg2);
return;
default:
g_assert_not_reached();
@@ -2497,8 +2502,8 @@ static void tcg_out_st(TCGContext *s, TCGType type, TCGReg arg,
tcg_out_vldst(s, INSN_VST1 | 0x7d0, arg, arg1, arg2);
return;
case TCG_TYPE_V128:
- /* regs 2; size 8; align 16 */
- tcg_out_vldst(s, INSN_VST1 | 0xae0, arg, arg1, arg2);
+ /* See tcg_out_ld re alignment: regs 2; size 8; align 8 */
+ tcg_out_vldst(s, INSN_VST1 | 0xad0, arg, arg1, arg2);
return;
default:
g_assert_not_reached();
--
2.25.1
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON
2021-09-12 17:49 [PATCH] tcg/arm: Reduce vector alignment requirement for NEON Richard Henderson
@ 2021-09-13 8:07 ` Richard W.M. Jones
2021-09-13 11:07 ` Richard W.M. Jones
1 sibling, 0 replies; 6+ messages in thread
From: Richard W.M. Jones @ 2021-09-13 8:07 UTC (permalink / raw)
To: Richard Henderson; +Cc: peter.maydell, qemu-devel
On Sun, Sep 12, 2021 at 10:49:25AM -0700, Richard Henderson wrote:
> With arm32, the ABI gives us 8-byte alignment for the stack.
> While it's possible to realign the stack to provide 16-byte alignment,
> it's far easier to simply not encode 16-byte alignment in the
> VLD1 and VST1 instructions that we emit.
>
> Remove the assertion in temp_allocate_frame, limit natural alignment
> to the provided stack alignment, and add a comment.
>
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>
> I haven't seen the assertion with the various arm kernels that I happen
> to have laying about. I have not taken the time to build the combo
> from the bug report:
>
> [ 0.000000] Linux version 5.14.0-60.fc36.armv7hl (mockbuild@buildvm-a32-12.iad2.fedoraproject.org) (gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1), GNU ld version 2.37-9.fc36) #1 SMP Mon Aug 30 14:08:34 UTC 2021
>
> I thought about parameterizing this patch further, but I can't think of
> another ISA that would be affected. (i686 clumsily changed its abi 20
> years ago to avoid faulting on vector spills; other isas so far have
> allowed vectors to be unaligned.)
You should be able to download the Fedora kernel that I am using from
https://koji.fedoraproject.org/koji/packageinfo?packageID=8
I added the patch to Fedora qemu and will do a test build once it
becomes available to build against - currently there's some problem
with new builds not propagating to the new buildroot.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON
2021-09-12 17:49 [PATCH] tcg/arm: Reduce vector alignment requirement for NEON Richard Henderson
2021-09-13 8:07 ` Richard W.M. Jones
@ 2021-09-13 11:07 ` Richard W.M. Jones
2021-09-13 16:19 ` Richard Henderson
1 sibling, 1 reply; 6+ messages in thread
From: Richard W.M. Jones @ 2021-09-13 11:07 UTC (permalink / raw)
To: Richard Henderson; +Cc: peter.maydell, berrange, qemu-devel
On Sun, Sep 12, 2021 at 10:49:25AM -0700, Richard Henderson wrote:
> With arm32, the ABI gives us 8-byte alignment for the stack.
> While it's possible to realign the stack to provide 16-byte alignment,
> it's far easier to simply not encode 16-byte alignment in the
> VLD1 and VST1 instructions that we emit.
>
> Remove the assertion in temp_allocate_frame, limit natural alignment
> to the provided stack alignment, and add a comment.
>
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>
> I haven't seen the assertion with the various arm kernels that I happen
> to have laying about. I have not taken the time to build the combo
> from the bug report:
>
> [ 0.000000] Linux version 5.14.0-60.fc36.armv7hl (mockbuild@buildvm-a32-12.iad2.fedoraproject.org) (gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1), GNU ld version 2.37-9.fc36) #1 SMP Mon Aug 30 14:08:34 UTC 2021
>
> I thought about parameterizing this patch further, but I can't think of
> another ISA that would be affected. (i686 clumsily changed its abi 20
> years ago to avoid faulting on vector spills; other isas so far have
> allowed vectors to be unaligned.)
Is it possible this change could have caused a more serious
regression? Now when I try to boot the Fedora kernel using TCG on
armv7hl I can't even get to the point where it detects virtio-scsi
devices.
Full log is here (go down to the bottom and work backwards):
https://kojipkgs.fedoraproject.org//work/tasks/7337/75597337/build.log
This might have been caused by a coincidental change to the kernel.
The test environment I have makes it extremely difficult to test this
change in isolation.
However I do know that the same error does _not_ occur on x86-64
guest/host with this patch applied.
Rich.
>
> r~
> ---
> tcg/tcg.c | 8 +++++++-
> tcg/arm/tcg-target.c.inc | 13 +++++++++----
> 2 files changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 4142d42d77..ca5bcc4635 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -3060,7 +3060,13 @@ static void temp_allocate_frame(TCGContext *s, TCGTemp *ts)
> g_assert_not_reached();
> }
>
> - assert(align <= TCG_TARGET_STACK_ALIGN);
> + /*
> + * Assume the stack is sufficiently aligned.
> + * This affects e.g. ARM NEON, where we have 8 byte stack alignment
> + * and do not require 16 byte vector alignment. This seems slightly
> + * easier than fully parameterizing the above switch statement.
> + */
> + align = MIN(TCG_TARGET_STACK_ALIGN, align);
> off = ROUND_UP(s->current_frame_offset, align);
>
> /* If we've exhausted the stack frame, restart with a smaller TB. */
> diff --git a/tcg/arm/tcg-target.c.inc b/tcg/arm/tcg-target.c.inc
> index e5b4f86841..8515717435 100644
> --- a/tcg/arm/tcg-target.c.inc
> +++ b/tcg/arm/tcg-target.c.inc
> @@ -2477,8 +2477,13 @@ static void tcg_out_ld(TCGContext *s, TCGType type, TCGReg arg,
> tcg_out_vldst(s, INSN_VLD1 | 0x7d0, arg, arg1, arg2);
> return;
> case TCG_TYPE_V128:
> - /* regs 2; size 8; align 16 */
> - tcg_out_vldst(s, INSN_VLD1 | 0xae0, arg, arg1, arg2);
> + /*
> + * We have only 8-byte alignment for the stack per the ABI.
> + * Rather than dynamically re-align the stack, it's easier
> + * to simply not request alignment beyond that. So:
> + * regs 2; size 8; align 8
> + */
> + tcg_out_vldst(s, INSN_VLD1 | 0xad0, arg, arg1, arg2);
> return;
> default:
> g_assert_not_reached();
> @@ -2497,8 +2502,8 @@ static void tcg_out_st(TCGContext *s, TCGType type, TCGReg arg,
> tcg_out_vldst(s, INSN_VST1 | 0x7d0, arg, arg1, arg2);
> return;
> case TCG_TYPE_V128:
> - /* regs 2; size 8; align 16 */
> - tcg_out_vldst(s, INSN_VST1 | 0xae0, arg, arg1, arg2);
> + /* See tcg_out_ld re alignment: regs 2; size 8; align 8 */
> + tcg_out_vldst(s, INSN_VST1 | 0xad0, arg, arg1, arg2);
> return;
> default:
> g_assert_not_reached();
> --
> 2.25.1
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON
2021-09-13 11:07 ` Richard W.M. Jones
@ 2021-09-13 16:19 ` Richard Henderson
2021-09-13 16:28 ` Richard W.M. Jones
0 siblings, 1 reply; 6+ messages in thread
From: Richard Henderson @ 2021-09-13 16:19 UTC (permalink / raw)
To: Richard W.M. Jones; +Cc: peter.maydell, berrange, qemu-devel
On 9/13/21 4:07 AM, Richard W.M. Jones wrote:
>> [ 0.000000] Linux version 5.14.0-60.fc36.armv7hl (mockbuild@buildvm-a32-12.iad2.fedoraproject.org) (gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1), GNU ld version 2.37-9.fc36) #1 SMP Mon Aug 30 14:08:34 UTC 2021
>>
>> I thought about parameterizing this patch further, but I can't think of
>> another ISA that would be affected. (i686 clumsily changed its abi 20
>> years ago to avoid faulting on vector spills; other isas so far have
>> allowed vectors to be unaligned.)
>
> Is it possible this change could have caused a more serious
> regression?
I don't think so...
> Now when I try to boot the Fedora kernel using TCG on
> armv7hl I can't even get to the point where it detects virtio-scsi
> devices.
>
> Full log is here (go down to the bottom and work backwards):
>
> https://kojipkgs.fedoraproject.org//work/tasks/7337/75597337/build.log
I downloaded the 5.14.0-60 kernel from above, and I can reproduce the original error, and
I see that it's fixed afterward.
I'll have a look at this new build log in a moment...
r~
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON
2021-09-13 16:19 ` Richard Henderson
@ 2021-09-13 16:28 ` Richard W.M. Jones
2021-09-13 17:01 ` Peter Maydell
0 siblings, 1 reply; 6+ messages in thread
From: Richard W.M. Jones @ 2021-09-13 16:28 UTC (permalink / raw)
To: Richard Henderson; +Cc: peter.maydell, berrange, qemu-devel
On Mon, Sep 13, 2021 at 09:19:22AM -0700, Richard Henderson wrote:
> On 9/13/21 4:07 AM, Richard W.M. Jones wrote:
> >>[ 0.000000] Linux version 5.14.0-60.fc36.armv7hl (mockbuild@buildvm-a32-12.iad2.fedoraproject.org) (gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1), GNU ld version 2.37-9.fc36) #1 SMP Mon Aug 30 14:08:34 UTC 2021
> >>
> >>I thought about parameterizing this patch further, but I can't think of
> >>another ISA that would be affected. (i686 clumsily changed its abi 20
> >>years ago to avoid faulting on vector spills; other isas so far have
> >>allowed vectors to be unaligned.)
> >
> >Is it possible this change could have caused a more serious
> >regression?
>
> I don't think so...
>
> >Now when I try to boot the Fedora kernel using TCG on
> >armv7hl I can't even get to the point where it detects virtio-scsi
> >devices.
> >
> >Full log is here (go down to the bottom and work backwards):
> >
> > https://kojipkgs.fedoraproject.org//work/tasks/7337/75597337/build.log
>
> I downloaded the 5.14.0-60 kernel from above, and I can reproduce
> the original error, and I see that it's fixed afterward.
Thanks for checking that. I did spend much of today attempting to get
an armv7 guest installed on my RPi so I could reproduce this more
reliably, but eventually gave up because of a variety of other
problems (one apparently in qemu: https://bugzilla.redhat.com/1633328)
> I'll have a look at this new build log in a moment...
>
>
> r~
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines. Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON
2021-09-13 16:28 ` Richard W.M. Jones
@ 2021-09-13 17:01 ` Peter Maydell
0 siblings, 0 replies; 6+ messages in thread
From: Peter Maydell @ 2021-09-13 17:01 UTC (permalink / raw)
To: Richard W.M. Jones; +Cc: Daniel P. Berrange, Richard Henderson, QEMU Developers
On Mon, 13 Sept 2021 at 17:28, Richard W.M. Jones <rjones@redhat.com> wrote:
> Thanks for checking that. I did spend much of today attempting to get
> an armv7 guest installed on my RPi so I could reproduce this more
> reliably, but eventually gave up because of a variety of other
> problems (one apparently in qemu: https://bugzilla.redhat.com/1633328)
That bug looks like it's the old "if you have an old non-LPAE
guest kernel you need to tell QEMU '-machine highmem=off' now".
So you can either do that, or use an LPAE kernel...
-- PMM
^ permalink raw reply [flat|nested] 6+ messages in thread
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2021-09-12 17:49 [PATCH] tcg/arm: Reduce vector alignment requirement for NEON Richard Henderson
2021-09-13 8:07 ` Richard W.M. Jones
2021-09-13 11:07 ` Richard W.M. Jones
2021-09-13 16:19 ` Richard Henderson
2021-09-13 16:28 ` Richard W.M. Jones
2021-09-13 17:01 ` Peter Maydell
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