linux-kernel - Re: [PATCH 11/28] KVM: SVM: Add helpers for accessing MSR bitmap that don't rely on offsets

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Message-ID: <1392db34-0c37-49db-8ece-68c02ff3520d@redhat.com>
Date: Wed, 4 Jun 2025 18:11:40 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Borislav Petkov <bp@...en8.de>, Xin Li <xin@...or.com>,
 Chao Gao <chao.gao@...el.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [PATCH 11/28] KVM: SVM: Add helpers for accessing MSR bitmap that
 don't rely on offsets

On 5/30/25 01:39, Sean Christopherson wrote:
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 47a36a9a7fe5..e432cd7a7889 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -628,6 +628,50 @@ static_assert(SVM_MSRS_PER_RANGE == 8192);
>   #define SVM_MSRPM_RANGE_1_BASE_MSR	0xc0000000
>   #define SVM_MSRPM_RANGE_2_BASE_MSR	0xc0010000
>   
> +#define SVM_MSRPM_FIRST_MSR(range_nr)	\
> +	(SVM_MSRPM_RANGE_## range_nr ##_BASE_MSR)
> +#define SVM_MSRPM_LAST_MSR(range_nr)	\
> +	(SVM_MSRPM_RANGE_## range_nr ##_BASE_MSR + SVM_MSRS_PER_RANGE - 1)
> +
> +#define SVM_MSRPM_BIT_NR(range_nr, msr)						\
> +	(range_nr * SVM_MSRPM_BYTES_PER_RANGE * BITS_PER_BYTE +			\
> +	 (msr - SVM_MSRPM_RANGE_## range_nr ##_BASE_MSR) * SVM_BITS_PER_MSR)
> +
> +#define SVM_MSRPM_SANITY_CHECK_BITS(range_nr)					\
> +static_assert(SVM_MSRPM_BIT_NR(range_nr, SVM_MSRPM_FIRST_MSR(range_nr) + 1) ==	\
> +	      range_nr * 2048 * 8 + 2);						\
> +static_assert(SVM_MSRPM_BIT_NR(range_nr, SVM_MSRPM_FIRST_MSR(range_nr) + 7) ==	\
> +	      range_nr * 2048 * 8 + 14);
> +
> +SVM_MSRPM_SANITY_CHECK_BITS(0);
> +SVM_MSRPM_SANITY_CHECK_BITS(1);
> +SVM_MSRPM_SANITY_CHECK_BITS(2);

Replying here for patches 11/25/26.  None of this is needed, just write 
a function like this:

static inline u32 svm_msr_bit(u32 msr)
{
	u32 msr_base = msr & ~(SVM_MSRS_PER_RANGE - 1);
	if (msr_base == SVM_MSRPM_RANGE_0_BASE_MSR)
		return SVM_MSRPM_BIT_NR(0, msr);
	if (msr_base == SVM_MSRPM_RANGE_1_BASE_MSR)
		return SVM_MSRPM_BIT_NR(1, msr);
	if (msr_base == SVM_MSRPM_RANGE_2_BASE_MSR)
		return SVM_MSRPM_BIT_NR(2, msr);
	return MSR_INVALID;
}

and you can throw away most of the other macros.  For example:

> +#define SVM_BUILD_MSR_BITMAP_CASE(bitmap, range_nr, msr, bitop, bit_rw)		\
> +	case SVM_MSRPM_FIRST_MSR(range_nr) ... SVM_MSRPM_LAST_MSR(range_nr):	\
> +		return bitop##_bit(SVM_MSRPM_BIT_NR(range_nr, msr) + bit_rw, bitmap);

... becomes a lot more lowercase:

static inline rtype svm_##action##_msr_bitmap_##access(
	unsigned long *bitmap, u32 msr)
{
	u32 bit = svm_msr_bit(msr);
	if (bit == MSR_INVALID)
		return true;
	return bitop##_bit(bit + bit_rw, bitmap);
}


In patch 25, also, you just get

static u32 svm_msrpm_offset(u32 msr)
{
	u32 bit = svm_msr_bit(msr);
	if (bit == MSR_INVALID)
		return MSR_INVALID;
	return bit / BITS_PER_BYTE;
}

And you change everything to -EINVAL in patch 26 to kill MSR_INVALID.

Another nit...

> +#define BUILD_SVM_MSR_BITMAP_HELPERS(ret_type, action, bitop)			\
> +	__BUILD_SVM_MSR_BITMAP_HELPER(ret_type, action, bitop, read,  0)	\
> +	__BUILD_SVM_MSR_BITMAP_HELPER(ret_type, action, bitop, write, 1)
> +
> +BUILD_SVM_MSR_BITMAP_HELPERS(bool, test, test)
> +BUILD_SVM_MSR_BITMAP_HELPERS(void, clear, __clear)
> +BUILD_SVM_MSR_BITMAP_HELPERS(void, set, __set)
Yes it's a bit duplication, but no need for the nesting, just do:

BUILD_SVM_MSR_BITMAP_HELPERS(bool, test,  test,    read,  0)
BUILD_SVM_MSR_BITMAP_HELPERS(bool, test,  test,    write, 1)
BUILD_SVM_MSR_BITMAP_HELPERS(void, clear, __clear, read,  0)
BUILD_SVM_MSR_BITMAP_HELPERS(void, clear, __clear, write, 1)
BUILD_SVM_MSR_BITMAP_HELPERS(void, set,   __set,   read,  0)
BUILD_SVM_MSR_BITMAP_HELPERS(void, set,   __set,   write, 1)

Otherwise, really nice.

Paolo