linux-kernel - Re: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

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Date:   Thu, 16 Jun 2022 14:02:32 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

On Wed, Jun 15, 2022 at 06:57:06PM +0000, Raghavendra Rao Ananta wrote:
> The selftests, when built with newer versions of clang, is found
> to have over optimized guests' ucall() function, and eliminating
> the stores for uc.cmd (perhaps due to no immediate readers). This
> resulted in the userspace side always reading a value of '0', and
> causing multiple test failures.
> 
> As a result, prevent the compiler from optimizing the stores in
> ucall() with WRITE_ONCE().
> 
> Suggested-by: Ricardo Koller <ricarkol@...gle.com>
> Suggested-by: Reiji Watanabe <reijiw@...gle.com>
> Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> ---
>  tools/testing/selftests/kvm/lib/aarch64/ucall.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/aarch64/ucall.c b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
> index e0b0164e9af8..be1d9728c4ce 100644
> --- a/tools/testing/selftests/kvm/lib/aarch64/ucall.c
> +++ b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
> @@ -73,20 +73,19 @@ void ucall_uninit(struct kvm_vm *vm)
>  
>  void ucall(uint64_t cmd, int nargs, ...)
>  {
> -	struct ucall uc = {
> -		.cmd = cmd,
> -	};
> +	struct ucall uc = {};
>  	va_list va;
>  	int i;
>  
> +	WRITE_ONCE(uc.cmd, cmd);
>  	nargs = nargs <= UCALL_MAX_ARGS ? nargs : UCALL_MAX_ARGS;
>  
>  	va_start(va, nargs);
>  	for (i = 0; i < nargs; ++i)
> -		uc.args[i] = va_arg(va, uint64_t);
> +		WRITE_ONCE(uc.args[i], va_arg(va, uint64_t));
>  	va_end(va);
>  
> -	*ucall_exit_mmio_addr = (vm_vaddr_t)&uc;
> +	WRITE_ONCE(*ucall_exit_mmio_addr, (vm_vaddr_t)&uc);
>  }
>  
>  uint64_t get_ucall(struct kvm_vm *vm, uint32_t vcpu_id, struct ucall *uc)
> -- 
> 2.36.1.476.g0c4daa206d-goog
>

Reviewed-by: Andrew Jones <drjones@...hat.com>

Thanks,
drew