Re: [PATCH v2] KVM: selftests: x86: Prioritize getting max_gfn from GuestPhysBits

[Xiaoyao Li <xiaoyao.li@xxxxxxxxx>]

On 5/10/2024 10:03 AM, Tao Su wrote:
> Use the max mappable GPA via GuestPhysBits advertised by KVM to calculate
> max_gfn. Currently some selftests (e.g. access_tracking_perf_test,
> dirty_log_test...) add RAM regions close to max_gfn, so guest may access
> GPA beyond its mappable range and cause infinite loop.
>
> Adjust max_gfn in vm_compute_max_gfn() since x86 selftests already
> overrides vm_compute_max_gfn() specifically to deal with goofy edge cases.
>
> Signed-off-by: Tao Su <tao1.su@xxxxxxxxxxxxxxx>
> Tested-by: Yi Lai <yi1.lai@xxxxxxxxx>
> ---
> This patch is based on https://github.com/kvm-x86/linux/commit/b628cb523c65
>
> Changelog:
> v1 -> v2:
>   - Only adjust vm->max_gfn in vm_compute_max_gfn()
>   - Add Yi Lai's Tested-by
>
> v1: https://lore.kernel.org/all/20240508064205.15301-1-tao1.su@xxxxxxxxxxxxxxx/
> ---
>   tools/testing/selftests/kvm/include/x86_64/processor.h |  1 +
>   tools/testing/selftests/kvm/lib/x86_64/processor.c     | 10 ++++++++--
>   2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
> index 81ce37ec407d..ff99f66d81a0 100644
> --- a/tools/testing/selftests/kvm/include/x86_64/processor.h
> +++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
> @@ -282,6 +282,7 @@ struct kvm_x86_cpu_property {
>   #define X86_PROPERTY_MAX_EXT_LEAF		KVM_X86_CPU_PROPERTY(0x80000000, 0, EAX, 0, 31)
>   #define X86_PROPERTY_MAX_PHY_ADDR		KVM_X86_CPU_PROPERTY(0x80000008, 0, EAX, 0, 7)
>   #define X86_PROPERTY_MAX_VIRT_ADDR		KVM_X86_CPU_PROPERTY(0x80000008, 0, EAX, 8, 15)
> +#define X86_PROPERTY_MAX_GUEST_PHY_ADDR		KVM_X86_CPU_PROPERTY(0x80000008, 0, EAX, 16, 23)
>   #define X86_PROPERTY_SEV_C_BIT			KVM_X86_CPU_PROPERTY(0x8000001F, 0, EBX, 0, 5)
>   #define X86_PROPERTY_PHYS_ADDR_REDUCTION	KVM_X86_CPU_PROPERTY(0x8000001F, 0, EBX, 6, 11)
>   
> diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> index 74a4c736c9ae..aa9966ead543 100644
> --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> @@ -1293,10 +1293,16 @@ const struct kvm_cpuid2 *vcpu_get_supported_hv_cpuid(struct kvm_vcpu *vcpu)
>   unsigned long vm_compute_max_gfn(struct kvm_vm *vm)
>   {
>   	const unsigned long num_ht_pages = 12 << (30 - vm->page_shift); /* 12 GiB */
> -	unsigned long ht_gfn, max_gfn, max_pfn;
> +	unsigned long ht_gfn, max_gfn, max_pfn, max_bits = 0;
>   	uint8_t maxphyaddr;
>   
> -	max_gfn = (1ULL << (vm->pa_bits - vm->page_shift)) - 1;
> +	if (kvm_cpu_has_p(X86_PROPERTY_MAX_GUEST_PHY_ADDR))
> +		max_bits = kvm_cpu_property(X86_PROPERTY_MAX_GUEST_PHY_ADDR);

We can get rid of the kvm_cpu_has_p(X86_PROPERTY_MAX_GUEST_PHY_ADDR) 
check and call kvm_cpu_property() unconditionally. As a bonus, we don't 
need to init max_bits as 0.

BTW, could we just name it guest_pa_bits?

Otherwise,

Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>

> +
> +	if (!max_bits)
> +		max_bits = vm->pa_bits;
> +
> +	max_gfn = (1ULL << (max_bits - vm->page_shift)) - 1;
>   
>   	/* Avoid reserved HyperTransport region on AMD processors.  */
>   	if (!host_cpu_is_amd)
>
> base-commit: 448b3fe5a0eab5b625a7e15c67c7972169e47ff8