linux-kernel - Re: [PATCH v2 2/8] KVM: selftests: Add pmu.h for PMU events and common masks

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Date:   Wed, 28 Jun 2023 13:02:53 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jinrong Liang <ljr.kernel@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Like Xu <likexu@...cent.com>,
        David Matlack <dmatlack@...gle.com>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jinrong Liang <cloudliang@...cent.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/8] KVM: selftests: Add pmu.h for PMU events and
 common masks

On Tue, May 30, 2023, Jinrong Liang wrote:
> From: Jinrong Liang <cloudliang@...cent.com>
> 
> To introduce a new pmu.h header file under
> tools/testing/selftests/kvm/include/x86_64 directory to better
> organize the PMU performance event constants and common masks.
> It will enhance the maintainability and readability of the KVM
> selftests code.
> 
> In the new pmu.h header, to define the PMU performance events and
> masks that are relevant for x86_64, allowing developers to easily
> reference them and minimize potential errors in code that handles
> these values.

Same feedback as the previous changelog.

> Signed-off-by: Jinrong Liang <cloudliang@...cent.com>
> ---
>  .../selftests/kvm/include/x86_64/pmu.h        | 56 +++++++++++++++++++
>  1 file changed, 56 insertions(+)
>  create mode 100644 tools/testing/selftests/kvm/include/x86_64/pmu.h
> 
> diff --git a/tools/testing/selftests/kvm/include/x86_64/pmu.h b/tools/testing/selftests/kvm/include/x86_64/pmu.h
> new file mode 100644
> index 000000000000..0e0111b11024
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/include/x86_64/pmu.h
> @@ -0,0 +1,56 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * tools/testing/selftests/kvm/include/x86_64/pmu.h
> + *
> + * Copyright (C) 2023, Tencent, Inc.
> + */
> +#ifndef _PMU_H_
> +#define _PMU_H_

SELFTEST_KVM_PMU_H for consistency, and to minimize the risk of a collision.

> +#include "processor.h"
> +
> +#define GP_CTR_NUM_OFS_BIT 8
> +#define EVT_LEN_OFS_BIT 24

Please spell out the words, I genuinely have no idea what these refer to, and
readers shouldn't have to consult the SDM just to understand a name.

> +#define INTEL_PMC_IDX_FIXED 32
> +
> +#define PMU_CAP_FW_WRITES BIT_ULL(13)
> +#define EVENTSEL_OS BIT_ULL(17)
> +#define EVENTSEL_ANY BIT_ULL(21)
> +#define EVENTSEL_EN BIT_ULL(22)
> +#define RDPMC_FIXED_BASE BIT_ULL(30)
> +
> +#define PMU_VERSION_MASK GENMASK_ULL(7, 0)
> +#define EVENTS_MASK GENMASK_ULL(7, 0)
> +#define EVT_LEN_MASK GENMASK_ULL(31, EVT_LEN_OFS_BIT)
> +#define GP_CTR_NUM_MASK GENMASK_ULL(15, GP_CTR_NUM_OFS_BIT)
> +#define FIXED_CTR_NUM_MASK GENMASK_ULL(4, 0)
> +
> +#define X86_INTEL_PMU_VERSION		kvm_cpu_property(X86_PROPERTY_PMU_VERSION)
> +#define X86_INTEL_MAX_GP_CTR_NUM	kvm_cpu_property(X86_PROPERTY_PMU_NR_GP_COUNTERS)
> +#define X86_INTEL_MAX_FIXED_CTR_NUM	kvm_cpu_property(X86_PROPERTY_PMU_NR_FIXED_COUNTERS)
> +#define X86_INTEL_FIXED_CTRS_BITMASK	kvm_cpu_property(X86_PROPERTY_PMU_FIXED_CTRS_BITMASK)

Please don't add macros like this.  It gives the false impression that all these
values are constant at compile time, which is very much not the case.  I really,
really dislike code that hides important details, like the fact that this is
querying KVM.

Yeah, the line lengths will be longer, but 80 chars is a soft limit, and we can
always get creative, e.g.

	uint8_t max_pmu_version = kvm_cpu_property(X86_PROPERTY_PMU_VERSION);
	struct kvm_vm *vm;
	struct kvm_vcpu *vcpu;
	uint8_t version;

	TEST_REQUIRE(kvm_cpu_property(X86_PROPERTY_PMU_NR_FIXED_COUNTERS) > 2);

> +/* Definitions for Architectural Performance Events */
> +#define ARCH_EVENT(select, umask) (((select) & 0xff) | ((umask) & 0xff) << 8)
> +
> +/* Intel Pre-defined Architectural Performance Events */
> +static const uint64_t arch_events[] = {
> +	[0] = ARCH_EVENT(0x3c, 0x0),
> +	[1] = ARCH_EVENT(0xc0, 0x0),
> +	[2] = ARCH_EVENT(0x3c, 0x1),
> +	[3] = ARCH_EVENT(0x2e, 0x4f),
> +	[4] = ARCH_EVENT(0x2e, 0x41),
> +	[5] = ARCH_EVENT(0xc4, 0x0),
> +	[6] = ARCH_EVENT(0xc5, 0x0),
> +	[7] = ARCH_EVENT(0xa4, 0x1),

Please do something like I proposed for KVM, i.e. avoid magic numbers inasmuch
as possible.

https://lore.kernel.org/all/20230607010206.1425277-2-seanjc@google.com

> +};
> +
> +/* Association of Fixed Counters with Architectural Performance Events */
> +static int fixed_events[] = {1, 0, 7};
> +
> +static inline uint64_t evt_code_for_fixed_ctr(uint8_t idx)

s/evt/event.  Having consistent naming is more important than saving two characters.

> +{
> +	return arch_events[fixed_events[idx]];
> +}
> +
> +#endif /* _PMU_H_ */
> -- 
> 2.31.1
>