linux-kernel - Re: [V2 PATCH] x86/tdx: add VIRT_CPUID2 virtualization if REDUCE

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Message-ID: <sdczrx3z55vbf4jxbedjpccpdmz7h7ukhhiwrwblpozwyrqssv@ljszzrxvgoxr>
Date: Wed, 30 Apr 2025 12:33:55 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Zhiquan Li <zhiquan1.li@...el.com>
Cc: Dave Hansen <dave.hansen@...el.com>, Jun Miao <jun.miao@...el.com>, 
	dave.hansen@...ux.intel.com, x86@...nel.org, linux-coco@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	"Du, Fan" <fan.du@...el.com>
Subject: Re: [V2 PATCH] x86/tdx: add VIRT_CPUID2 virtualization if REDUCE_VE
 was not successful

On Wed, Apr 30, 2025 at 10:15:05AM +0800, Zhiquan Li wrote:
> 
> On 2025/4/29 22:50, Dave Hansen wrote:
> > On 4/29/25 07:31, Jun Miao wrote:
> >> REDUCE_VE can only be enabled if x2APIC_ID has been properly configured
> >> with unique values for each VCPU.  Check if VMM has provided an activated
> >> topology configuration first as it is the prerequisite of REDUCE_VE and
> >> ENUM_TOPOLOGY, so move it to reduce_unnecessary_ve().  The function
> >> enable_cpu_topology_enumeration() was very little and can be
> >> integrated into reduce_unnecessary_ve().
> > 
> > Isn't this just working around VMM bugs? Shouldn't we just panic as
> > quickly as possible so the VMM config gets fixed rather than adding kludges?
> 
> 
> Now failed to virtualize these two cases will cause TD VM regression vs
> legacy VM.  Do you mean the panic will just for the #VE caused by CPUID
> leaf 0x2? Or both (+ VMM not configure topology) will panic?
> 
> Currently the most customer's complaints come from the CPUID leaf 0x2
> not virtualization, and most of access come from user space.  Is it
> appropriate for such behavior directly cause a guest kernel panic?

The appropriate behavior would be to fix VMM to configure APIC IDs
correctly and use TDX module that supports REDUCE_VE.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov