linux-kernel - Re: [PATCH v2 2/7] KVM: x86: Extract VMXON and EFER.SVME enablement to kernel

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <e3905a0a-8582-4902-9ca7-03b6e3ce412c@linux.intel.com>
Date: Tue, 27 Jan 2026 10:46:30 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Xu Yilun <yilun.xu@...ux.intel.com>
Cc: Chao Gao <chao.gao@...el.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 Kiryl Shutsemau <kas@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
 linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev,
 kvm@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 2/7] KVM: x86: Extract VMXON and EFER.SVME enablement
 to kernel



On 12/19/2025 11:40 PM, Sean Christopherson wrote:
> On Fri, Dec 19, 2025, Xu Yilun wrote:
>> On Wed, Dec 17, 2025 at 11:01:59AM -0800, Sean Christopherson wrote:
>>> On Wed, Dec 17, 2025, Xu Yilun wrote:
>>>> Is it better we explicitly assert the preemption for x86_virt_get_cpu()
>>>> rather than embed the check in __this_cpu_inc_return()? We are not just
>>>> protecting the racing for the reference counter. We should ensure the
>>>> "counter increase + x86_virt_call(get_cpu)" can't be preempted.
>>>
>>> I don't have a strong preference.  Using __this_cpu_inc_return() without any
>>> nearby preemption_{enable,disable}() calls makes it quite clears that preemption
>>> is expected to be disabled by the caller.  But I'm also ok being explicit.
>>
>> Looking into __this_cpu_inc_return(), it finally calls
>> check_preemption_disabled() which doesn't strictly requires preemption.
>> It only ensures the context doesn't switch to another CPU. If the caller
>> is in cpuhp context, preemption is possible.
> 
> Hmm, right, the cpuhp thread is is_percpu_thread(), and KVM's hooks aren't
> considered atomic and so run with IRQs enabled.  In practice, it's "fine", because
> TDX also exclusively does x86_virt_get_cpu() from cpuhp, i.e. the two users are
> mutually exclusive, but relying on that behavior is gross.
> 

Side topic:
Isn't the name of check_preemption_disabled() confusing and arguably misleading?