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

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Message-ID: <0e298f80-5361-4376-a099-d3457c8cd349@intel.com>
Date: Wed, 30 Apr 2025 23:09:38 +0800
From: Zhiquan Li <zhiquan1.li@...el.com>
To: Dave Hansen <dave.hansen@...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 2025/4/30 21:44, Dave Hansen wrote:
>> 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.

OK, I agree with you.  Is it better to simplify the logic like this:

    static void reduce_unnecessary_ve(void)
    {
        u64 configured;

        /* Has the VMM provided a valid topology configuration? */
        tdg_vm_rd(TDCS_TOPOLOGY_ENUM_CONFIGURED, &configured);

        if (!configured)
            panic("VMM did not configure X2APIC_IDs properly\n");

        tdg_vm_wr(TDCS_TD_CTLS, TD_CTLS_REDUCE_VE, TD_CTLS_REDUCE_VE);
    }

Since merely fall back to enable ENUM_TOPOLOGY isn't enough, the guest
still suffering the CPUID leaf 0x2 not virtualization regression, like
the glibc bug.  Full REDUCE_VE is really expected.

Best Regards,
Zhiquan