Subject: Re: [kvm-unit-tests PATCH v1 4/5] arm/arm64: Add support for --processor=max

From: Alexandru Elisei <[email-masked]>
Date: Tue, 14 Jan 2025 17:20:13 +0000

Hi Drew,

On Mon, Jan 13, 2025 at 04:21:45PM +0100, Andrew Jones wrote:
> On Fri, Jan 10, 2025 at 01:58:47PM +0000, Alexandru Elisei wrote:
> > For arm64, newer architecture features are supported only on newer CPUs.
> > Instead of expecting the user to know which CPU model supports which
> > feature when using the TCG accelerator for qemu, let's make it easier and
> > add support for the --processor 'max' value.
> > 
> > The --processor value is passed to the compiler's -mcpu argument and to
> > qemu's -cpu argument. 'max' is a special value that only qemu understands -
> > it means that all CPU features that qemu implements are supported by the
> > guest CPU, and passing it to the compiler causes a build error. So omit the
> > -mcpu argument when $PROCESSOR=max.
> > 
> > This affects only the TCG accelerator; when using KVM or HVF,
> > kvm-unit-tests sets the cpu model to 'host'.
> > 
> > Note that using --processor=max with a 32 bit compiler will cause a build
> > error: the CPU model that the compiler defaults to when the -mcpu argument
> > is missing lacks support for some of the instructions that kvm-unit-tests
> > uses. The solution in the case is to specify a CPU model for the compiler
> > using --cflags:
> > 
> >   ./configure --arch=arm --processor=max --cflags=-mcpu=<cpu>
> > 
> > This patch doesn't introduce a regression for arm when --processor=max is
> > used, it's only the error that changes: from an unknown processor type to
> > using instructions that are not available on the processor.
> > 
> > Signed-off-by: Alexandru Elisei <[email-masked]>
> > ---
> >  arm/Makefile.common | 2 ++
> >  configure           | 5 ++++-
> >  2 files changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arm/Makefile.common b/arm/Makefile.common
> > index a5d97bcf477a..b757250dc9ae 100644
> > --- a/arm/Makefile.common
> > +++ b/arm/Makefile.common
> > @@ -25,7 +25,9 @@ AUXFLAGS ?= 0x0
> >  # stack.o relies on frame pointers.
> >  KEEP_FRAME_POINTER := y
> >  
> > +ifneq ($(PROCESSOR),max)
> >  CFLAGS += -mcpu=$(PROCESSOR)
> > +endif
> >  CFLAGS += -std=gnu99
> >  CFLAGS += -ffreestanding
> >  CFLAGS += -O2
> > diff --git a/configure b/configure
> > index 138840c3f76d..46964d36a7d8 100755
> > --- a/configure
> > +++ b/configure
> > @@ -67,7 +67,10 @@ usage() {
> >  	Options include:
> >  	    --arch=ARCH            architecture to compile for ($arch). ARCH can be one of:
> >  	                           arm, arm64/aarch64, i386, ppc64, riscv32, riscv64, s390x, x86_64
> > -	    --processor=PROCESSOR  processor to compile for ($default_processor)
> > +	    --processor=PROCESSOR  processor to compile for ($default_processor). For arm and arm64, the
> > +	                           value 'max' is special and it will be passed directly to
> > +	                           qemu, bypassing the compiler. In this case, --cflags can be
> > +	                           used to compile for a specific processor.
> >  	    --target=TARGET        target platform that the tests will be running on (qemu or
> >  	                           kvmtool, default is qemu) (arm/arm64 only)
> >  	    --cross-prefix=PREFIX  cross compiler prefix
> > -- 
> > 2.47.1
> >
> 
> I don't think we want to overload processor this way. While mcpu and QEMU
> could both understand the same cpu names, then it was mostly fine
> (although it probably shouldn't have been overloaded before either). Now
> that we want one name for compiling and another for running, then I think
> we need another configure parameter, something like --qemu-cpu.

I agree, that's a better approach than overloading --processor. I'll try that
for the next iteration.

Thanks,
Alex