linux-kernel - RE: [PATCH 0/6] x86/msr: let paravirt inline rdmsr/wrmsr instructions

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Message-ID:
 <SN6PR02MB415797F46C1F8F7CCEEFFFE9D495A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Sat, 10 May 2025 16:03:54 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Juergen Gross <jgross@...e.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "virtualization@...ts.linux.dev"
	<virtualization@...ts.linux.dev>
CC: "xin@...or.com" <xin@...or.com>, "Kirill A. Shutemov"
	<kirill.shutemov@...ux.intel.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, Sean
 Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, "K.
 Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, Vitaly
 Kuznetsov <vkuznets@...hat.com>, Boris Ostrovsky
	<boris.ostrovsky@...cle.com>, "xen-devel@...ts.xenproject.org"
	<xen-devel@...ts.xenproject.org>, Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.amakhalov@...adcom.com>, Broadcom internal kernel
 review list <bcm-kernel-feedback-list@...adcom.com>
Subject: RE: [PATCH 0/6] x86/msr: let paravirt inline rdmsr/wrmsr instructions

From: Juergen Gross <jgross@...e.com> Sent: Tuesday, May 6, 2025 2:20 AM
> 
> When building a kernel with CONFIG_PARAVIRT_XXL the paravirt
> infrastructure will always use functions for reading or writing MSRs,
> even when running on bare metal.
> 
> Switch to inline RDMSR/WRMSR instructions in this case, reducing the
> paravirt overhead.
> 
> In order to make this less intrusive, some further reorganization of
> the MSR access helpers is done in the first 4 patches.
> 
> This series has been tested to work with Xen PV and on bare metal.

I've tested in SEV-SNP and TDX guests with paravisor on Hyper-V. Basic
smoke test showed no issues.

Tested-by: Michael Kelley <mhklinux@...look.com>

> 
> There has been another approach by Xin Li, which used dedicated #ifdef
> and removing the MSR related paravirt hooks instead of just modifying
> the paravirt code generation.
> 
> Please note that I haven't included the use of WRMSRNS or the
> immediate forms of WRMSR and RDMSR, because I wanted to get some
> feedback on my approach first. Enhancing paravirt for those cases
> is not very complicated, as the main base is already prepared for
> that enhancement.
> 
> This series is based on the x86/msr branch of the tip tree.
> 
> Juergen Gross (6):
>   coco/tdx: Rename MSR access helpers
>   x86/kvm: Rename the KVM private read_msr() function
>   x86/msr: minimize usage of native_*() msr access functions
>   x86/msr: Move MSR trace calls one function level up
>   x86/paravirt: Switch MSR access pv_ops functions to instruction
>     interfaces
>   x86/msr: reduce number of low level MSR access helpers
> 
>  arch/x86/coco/tdx/tdx.c                   |   8 +-
>  arch/x86/hyperv/ivm.c                     |   2 +-
>  arch/x86/include/asm/kvm_host.h           |   2 +-
>  arch/x86/include/asm/msr.h                | 116 ++++++++++-------
>  arch/x86/include/asm/paravirt.h           | 152 ++++++++++++++--------
>  arch/x86/include/asm/paravirt_types.h     |  13 +-
>  arch/x86/include/asm/qspinlock_paravirt.h |   5 +-
>  arch/x86/kernel/kvmclock.c                |   2 +-
>  arch/x86/kernel/paravirt.c                |  26 +++-
>  arch/x86/kvm/svm/svm.c                    |  16 +--
>  arch/x86/kvm/vmx/vmx.c                    |   4 +-
>  arch/x86/xen/enlighten_pv.c               |  60 ++++++---
>  arch/x86/xen/pmu.c                        |   4 +-
>  13 files changed, 262 insertions(+), 148 deletions(-)
> 
> --
> 2.43.0
>