linux-kernel - Re: [PATCH v3 06/27] x86/cea: Export per CPU variable cea_exception

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Message-ID: <93b3d510-f21b-4f89-ae53-0fa50f03a42d@intel.com>
Date: Tue, 1 Oct 2024 09:12:32 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Xin Li (Intel)" <xin@...or.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Cc: seanjc@...gle.com, pbonzini@...hat.com, corbet@....net,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, luto@...nel.org,
 peterz@...radead.org, andrew.cooper3@...rix.com
Subject: Re: [PATCH v3 06/27] x86/cea: Export per CPU variable
 cea_exception_stacks

On 9/30/24 22:00, Xin Li (Intel) wrote:
> The per CPU variable cea_exception_stacks contains per CPU stacks for
> NMI, #DB and #DF, which is referenced in KVM to set host FRED RSP[123]
> each time a vCPU is loaded onto a CPU, thus it needs to be exported.

Nit: It's not obvious how 'cea_exception_stacks' get used in this
series.  It's never referenced explicitly.

I did figure it out by looking for 'RSP[123]' references, but a much
better changelog would be something like:

	The per CPU array 'cea_exception_stacks' points to per CPU
	stacks for NMI, #DB and #DF. It is normally referenced via the
	#define: __this_cpu_ist_top_va().

	FRED introduced new fields in the host-state area of the VMCS
	for stack levels 1->3 (HOST_IA32_FRED_RSP[123]). KVM must
	populate these each time a vCPU is loaded onto a CPU.

See how that explicitly gives the reader greppable strings for
"__this_cpu_ist_top_va" and "HOST_IA32_FRED_RSP"?  That makes it much
easier to figure out what is going on.

I was also momentarily confused about why these loads need to be done on
_every_ vCPU load.  I think it's because the host state can change as
the vCPU moves around to different physical CPUs and
__this_cpu_ist_top_va() can and will change.  But it's a detail that I
think deserves to be explained in the changelog.  There is also this
note in vmx_vcpu_load_vmcs():

>                 /*
>                  * Linux uses per-cpu TSS and GDT, so set these when switching
>                  * processors.  See 22.2.4.
>                  */

which makes me think that it might not be bad to pull *all* of the
per-cpu VMCS field population code out into a helper since the reasoning
of why these need to be repopulated is identical.

Also, what's the purpose of clearing GUEST_IA32_FRED_RSP[123] at
init_vmcs() time?  I would have thought that those values wouldn't
matter until the VMCS gets loaded at vmx_vcpu_load_vmcs() when they are
overwritten anyway.  Or, I could be just totally misunderstanding how
KVM consumes the VMCS. :)