linux-kernel - Re: [PATCH v3 07/34] x86/hyperv: Introduce HV_MAX_SPARSE_VCPU_BANKS/HV_VCPUS_PER_SPARSE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <7b62c83b-e284-ce43-a229-2b7774edaf95@wanadoo.fr>
Date:   Mon, 25 Apr 2022 21:09:44 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 07/34] x86/hyperv: Introduce
 HV_MAX_SPARSE_VCPU_BANKS/HV_VCPUS_PER_SPARSE_BANK constants

Hi,

Le 14/04/2022 à 15:19, Vitaly Kuznetsov a écrit :
> It may not come clear from where the magical '64' value used in
> __cpumask_to_vpset() come from. Moreover, '64' means both the maximum
> sparse bank number as well as the number of vCPUs per bank. Add defines
> to make things clear. These defines are also going to be used by KVM.
> 
> No functional change.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>   include/asm-generic/hyperv-tlfs.h |  5 +++++
>   include/asm-generic/mshyperv.h    | 11 ++++++-----
>   2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
> index fdce7a4cfc6f..020ca9bdbb79 100644
> --- a/include/asm-generic/hyperv-tlfs.h
> +++ b/include/asm-generic/hyperv-tlfs.h
> @@ -399,6 +399,11 @@ struct hv_vpset {
>   	u64 bank_contents[];
>   } __packed;
>   
> +/* The maximum number of sparse vCPU banks which can be encoded by 'struct hv_vpset' */
> +#define HV_MAX_SPARSE_VCPU_BANKS (64)
> +/* The number of vCPUs in one sparse bank */
> +#define HV_VCPUS_PER_SPARSE_BANK (64)
> +
>   /* HvCallSendSyntheticClusterIpi hypercall */
>   struct hv_send_ipi {
>   	u32 vector;
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index c08758b6b364..0abe91df1ef6 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -214,9 +214,10 @@ static inline int __cpumask_to_vpset(struct hv_vpset *vpset,
>   {
>   	int cpu, vcpu, vcpu_bank, vcpu_offset, nr_bank = 1;
>   	int this_cpu = smp_processor_id();
> +	int max_vcpu_bank = hv_max_vp_index / HV_VCPUS_PER_SPARSE_BANK;
>   
> -	/* valid_bank_mask can represent up to 64 banks */
> -	if (hv_max_vp_index / 64 >= 64)
> +	/* vpset.valid_bank_mask can represent up to HV_MAX_SPARSE_VCPU_BANKS banks */
> +	if (max_vcpu_bank >= HV_MAX_SPARSE_VCPU_BANKS)
>   		return 0;
>   
>   	/*
> @@ -224,7 +225,7 @@ static inline int __cpumask_to_vpset(struct hv_vpset *vpset,
>   	 * structs are not cleared between calls, we risk flushing unneeded
>   	 * vCPUs otherwise.
>   	 */
> -	for (vcpu_bank = 0; vcpu_bank <= hv_max_vp_index / 64; vcpu_bank++)
> +	for (vcpu_bank = 0; vcpu_bank <= max_vcpu_bank; vcpu_bank++)
>   		vpset->bank_contents[vcpu_bank] = 0;
>   
>   	/*
> @@ -236,8 +237,8 @@ static inline int __cpumask_to_vpset(struct hv_vpset *vpset,
>   		vcpu = hv_cpu_number_to_vp_number(cpu);
>   		if (vcpu == VP_INVAL)
>   			return -1;
> -		vcpu_bank = vcpu / 64;
> -		vcpu_offset = vcpu % 64;
> +		vcpu_bank = vcpu / HV_VCPUS_PER_SPARSE_BANK;
> +		vcpu_offset = vcpu % HV_VCPUS_PER_SPARSE_BANK;
>   		__set_bit(vcpu_offset, (unsigned long *)
>   			  &vpset->bank_contents[vcpu_bank]);

Here, we could also use directly:
	__set_bit(vcpu, vpset->bank_contents);

This is simpler, more readable (IMHO) and also makes 'vcpu_offset' useless.
And in case gcc is not able to optimize it by itself, this should also 
save a few cycles.

Just my 2c,
CJ

>   		if (vcpu_bank >= nr_bank)