linux-kernel - Re: [Qemu-devel] arm64 qemu tests failing in linux-next since 'arm64: kernel: enforce pmuserenr

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Date:	Thu, 7 Jan 2016 15:53:19 +0000
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Peter Maydell <peter.maydell@...aro.org>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Rutland <mark.rutland@....com>,
	Will Deacon <will.deacon@....com>,
	QEMU Developers <qemu-devel@...gnu.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Qemu-devel] arm64 qemu tests failing in linux-next since
 'arm64: kernel: enforce pmuserenr_el0 initialization and restore'

On Thu, Jan 07, 2016 at 01:25:35PM +0000, Peter Maydell wrote:
> On 24 December 2015 at 00:52, Guenter Roeck <linux@...ck-us.net> wrote:
> > Hi all,
> >
> > since commit 60792ad349f3 ("arm64: kernel: enforce pmuserenr_el0
> > initialization
> > and restore"), my arm64 qemu tests of linux-next are failing. After this
> > commit,
> > qemu does not display any output.
> >
> > Qemu version is 2.5.0. Linux kernel configuration is arm64:defconfig.
> >
> > qemu command line is as follows:
> >
> >         qemu-system-aarch64 -machine virt -cpu cortex-a57 -machine type=virt
> > -nographic -smp 1 \
> >                 -m 512 -kernel arch/arm64/boot/Image -initrd
> > rootfs.arm64.cpio -no-reboot \
> >                 -append "console=ttyAMA0"
> >
> > Any idea what might cause this problem and how to fix it (presumably in
> > qemu) ?
> 
> This turns out to be because QEMU doesn't currently implement
> PMUSERENR_EL0 for AArch64 (we do have an AArch32 implementation),
> so you get an immediate UNDEF when the kernel touches it, followed
> by an infinite loop of UNDEF exceptions because the instruction
> at the UNDEF vector entrypoint is unallocated at this point in
> execution.
> 
> We had previously been relying on the kernel not attempting to
> touch the PMU if the ID_AA64DFR0_EL1 PMUVer bits read 0000
> ("Performance Monitors extension System registers not implemented").

Ok, thanks for looking into this. I wonder why reading pmcr_el0 does
not suffer from the same problem though.

> Since the v8 ARM ARM states that the Performance Monitors Extension is
> an optional feature of an implementation, this seems like a kernel
> bug to me. (QEMU should probably get round to implementing the PMU
> at some point for feature parity with v7, but this has not been
> a priority for us since they're not actually very useful in a
> fully emulated setup.)

Fixup patch coming, thanks.

Lorenzo
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