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

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Message-ID: <e8d3fe25-0f97-4edd-abad-dc6b96430965@intel.com>
Date: Wed, 30 Apr 2025 06:44:53 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Zhiquan Li <zhiquan1.li@...el.com>, Jun Miao <jun.miao@...el.com>,
 kirill.shutemov@...ux.intel.com, dave.hansen@...ux.intel.com
Cc: 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 4/29/25 19:15, 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?

I'm not really parsing this response. I don't understand what you are
asking.

> 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?

If a VMM doesn't properly configure topology properly and allows
REDUCE_VE, then panic. They obviously got something wrong because that
configuration doesn't make any sense.