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Re: [Qemu-arm] [PATCH for-4.1] target/arm: Limit ID register assertions

From: Richard Henderson
Subject: Re: [Qemu-arm] [PATCH for-4.1] target/arm: Limit ID register assertions to TCG
Date: Thu, 18 Jul 2019 07:58:26 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2

On 7/18/19 5:59 AM, Peter Maydell wrote:
> In arm_cpu_realizefn() we make several assertions about the values of
> guest ID registers:
>  * if the CPU provides AArch32 v7VE or better it must advertise the
>    ARM_DIV feature
>  * if the CPU provides AArch32 A-profile v6 or better it must
>    advertise the Jazelle feature
> These are essentially consistency checks that our ID register
> specifications in cpu.c didn't accidentally miss out a feature,
> because increasingly the TCG emulation gates features on the values
> in ID registers rather than using old-style checks of ARM_FEATURE_FOO
> bits.
> Unfortunately, these asserts can cause problems if we're running KVM,
> because in that case we don't control the values of the ID registers
> -- we read them from the host kernel.  In particular, if the host
> kernel is older than 4.15 then it doesn't expose the ID registers via
> the KVM_GET_ONE_REG ioctl, and we set up dummy values for some
> registers and leave the rest at zero.  (See the comment in
> target/arm/kvm64.c kvm_arm_get_host_cpu_features().) This set of
> dummy values is not sufficient to pass our assertions, and so on
> those kernels running an AArch32 guest on AArch64 will assert.
> We could provide a more sophisticated set of dummy ID registers in
> this case, but that still leaves the possibility of a host CPU which
> reports bogus ID register values that would cause us to assert.  It's
> more robust to only do these ID register checks if we're using TCG,
> as that is the only case where this is truly a QEMU code bug.
> Reported-by: Laszlo Ersek <address@hidden>
> Fixes: https://bugs.launchpad.net/qemu/+bug/1830864
> Signed-off-by: Peter Maydell <address@hidden>
> ---
> Laszlo, would you mind testing this on your setup? I don't have
> a system with an old enough kernel to trigger the assert. (The
> change is pretty much a "has to work" one though :-))

Reviewed-by: Richard Henderson <address@hidden>


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