linux-kernel - Re: [PATCH v4 04/19] x86/cea: Export per CPU array 'cea_exception

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Message-ID: <e88be442-1163-4163-8f2f-06a37d1e595a@intel.com>
Date: Thu, 10 Apr 2025 07:18:49 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Christoph Hellwig <hch@...radead.org>, "Xin Li (Intel)" <xin@...or.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, kvm@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, corbet@....net,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 andrew.cooper3@...rix.com, luto@...nel.org, peterz@...radead.org,
 chao.gao@...el.com, xin3.li@...el.com
Subject: Re: [PATCH v4 04/19] x86/cea: Export per CPU array
 'cea_exception_stacks' for KVM to use

On 4/10/25 01:53, Christoph Hellwig wrote:
> On Fri, Mar 28, 2025 at 10:11:50AM -0700, Xin Li (Intel) wrote:
>> The per CPU array 'cea_exception_stacks' points to per CPU stacks
>> +/*
>> + * FRED introduced new fields in the host-state area of the VMCS for
>> + * stack levels 1->3 (HOST_IA32_FRED_RSP[123]), each respectively
>> + * corresponding to per CPU stacks for #DB, NMI and #DF.  KVM must
>> + * populate these each time a vCPU is loaded onto a CPU.
>> + */
>> +EXPORT_PER_CPU_SYMBOL(cea_exception_stacks);
> Exporting data vs accessors for it is usually a bad idea.  Doing a
> non-_GPl for such a very low level data struture is even worse.

Big ack on this.

I don't even see a single caller of __this_cpu_ist_top_va() that's
remotely performance sensitive or that needs to be inline.

Just make the __this_cpu_ist_top/bottom_va() macros into real functions
and export __this_cpu_ist_top_va(). It's going to be a pretty tiny
function but I think that's tolerable.