linux-kernel - Re: [PATCH 2/2] KVM: selftests: Test forced instruction emulation in dirty log test (x86 only)

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Message-ID: <Zc3JcNVhghB0Chlz@linux.dev>
Date: Thu, 15 Feb 2024 08:21:04 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
	Pasha Tatashin <tatashin@...gle.com>,
	Michael Krebs <mkrebs@...gle.com>
Subject: Re: [PATCH 2/2] KVM: selftests: Test forced instruction emulation in
 dirty log test (x86 only)

On Wed, Feb 14, 2024 at 05:00:04PM -0800, Sean Christopherson wrote:
> Add forced emulation of MOV and LOCK CMPXCHG instructions in the dirty log
> test's guest code to verify that KVM's emulator marks pages dirty as
> expected (and obviously to verify the emulator works at all).  In the long
> term, the guest code would ideally hammer more of KVM's emulator, but for
> starters, cover the two major paths: writes and atomics.
> 
> To minimize #ifdeffery, wrap only the related code that is x86 specific,
> unnecessariliy synchronizing an extra boolean to the guest is far from the
> end of the world.

Meh, I wouldn't say the end result in guest_write_memory() is that
pretty. Just ifdef the whole function and provide a generic implementation
for the other architectures.

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  tools/testing/selftests/kvm/dirty_log_test.c | 36 ++++++++++++++++++--
>  1 file changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
> index eaad5b20854c..ff1d1c7f05d8 100644
> --- a/tools/testing/selftests/kvm/dirty_log_test.c
> +++ b/tools/testing/selftests/kvm/dirty_log_test.c
> @@ -92,6 +92,29 @@ static uint64_t guest_test_phys_mem;
>   */
>  static uint64_t guest_test_virt_mem = DEFAULT_GUEST_TEST_MEM;
>  
> +static bool is_forced_emulation_enabled;
> +
> +static void guest_write_memory(uint64_t *mem, uint64_t val, uint64_t rand)
> +{
> +#ifdef __x86_64__
> +	if (is_forced_emulation_enabled && (rand & 1)) {
> +		if (rand & 2) {

Can't you invert the logic and drop a level of indentation?

	if (!(is_forced_emulation_enabled && (rand & 1))) {
		*mem = val;
	} else if (rand & 2) {
		movq
	} else {
		cmpxchg8b
	}

> +			__asm__ __volatile__(KVM_FEP "movq %1, %0"
> +					     : "+m" (*mem)
> +					     : "r" (val) : "memory");
> +		} else {
> +			uint64_t __old = READ_ONCE(*mem);
> +
> +			__asm__ __volatile__(KVM_FEP LOCK_PREFIX "cmpxchgq %[new], %[ptr]"
> +					     : [ptr] "+m" (*mem), [old] "+a" (__old)
> +					     : [new]"r" (val) : "memory", "cc");
> +		}
> +	} else
> +#endif
> +
> +	*mem = val;
> +}
> +

-- 
Thanks,
Oliver