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

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Message-ID: <69352b2239a33_1b2e100d2@dwillia2-mobl4.notmuch>
Date: Sat, 6 Dec 2025 23:22:10 -0800
From: <dan.j.williams@...el.com>
To: Sean Christopherson <seanjc@...gle.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>, Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>
CC: <linux-kernel@...r.kernel.org>, <linux-coco@...ts.linux.dev>,
	<kvm@...r.kernel.org>, Chao Gao <chao.gao@...el.com>, Dan Williams
	<dan.j.williams@...el.com>
Subject: Re: [PATCH v2 2/7] KVM: x86: Extract VMXON and EFER.SVME enablement
 to kernel

Sean Christopherson wrote:
> Move the innermost VMXON and EFER.SVME management logic out of KVM and
> into to core x86 so that TDX can force VMXON without having to rely on
> KVM being loaded, e.g. to do SEAMCALLs during initialization.
> 
> Implement a per-CPU refcounting scheme so that "users", e.g. KVM and the
> future TDX code, can co-exist without pulling the rug out from under each
> other.
> 
> To avoid having to choose between SVM and VMX, simply refuse to enable
> either if both are somehow supported.  No known CPU supports both SVM and
> VMX, and it's comically unlikely such a CPU will ever exist.
> 
> For lack of a better name, call the new file "hw.c", to yield "virt
> hardware" when combined with its parent directory.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
[..]

Only patch organization and naming choice questions from me. I only
looked at the VMX side of things since I previously made an attempt at
moving that. I gave a cursory look at the SVM details.

Reviewed-by: Dan Williams <dan.j.williams@...el.com>

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 80cb882f19e2..f650f79d3d5a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -83,6 +83,8 @@
>  #include <asm/intel_pt.h>
>  #include <asm/emulate_prefix.h>
>  #include <asm/sgx.h>
> +#include <asm/virt.h>
> +
>  #include <clocksource/hyperv_timer.h>
>  
>  #define CREATE_TRACE_POINTS
> @@ -694,9 +696,6 @@ static void drop_user_return_notifiers(void)
>  		kvm_on_user_return(&msrs->urn);
>  }
>  
> -__visible bool kvm_rebooting;
> -EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_rebooting);

...a short stay for this symbol in kvm/x86.c? It raises my curiosity why
patch1 is separate. Patch1 looked like the start of a series of
incremental conversions, patch2 is a combo move. I am ok either way,
just questioning consistency. I.e. if combo move then patch1 folds in
here, if incremental, perhaps split out other combo conversions like
emergency_disable_virtualization_cpu()? The aspect of "this got moved
twice in the same patchset" is what poked me.

[..]
> diff --git a/arch/x86/virt/hw.c b/arch/x86/virt/hw.c
> new file mode 100644
> index 000000000000..986e780cf438
> --- /dev/null
> +++ b/arch/x86/virt/hw.c
> @@ -0,0 +1,340 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#include <linux/cpu.h>
> +#include <linux/cpumask.h>
> +#include <linux/errno.h>
> +#include <linux/kvm_types.h>
> +#include <linux/list.h>
> +#include <linux/percpu.h>
> +
> +#include <asm/perf_event.h>
> +#include <asm/processor.h>
> +#include <asm/virt.h>
> +#include <asm/vmx.h>
> +
> +static int x86_virt_feature __ro_after_init;
> +
> +__visible bool virt_rebooting;
> +EXPORT_SYMBOL_GPL(virt_rebooting);
> +
> +static DEFINE_PER_CPU(int, virtualization_nr_users);
> +
> +static cpu_emergency_virt_cb __rcu *kvm_emergency_callback;

Hmm, why kvm_ and not virt_?

[..]
> +#if IS_ENABLED(CONFIG_KVM_INTEL)
> +static DEFINE_PER_CPU(struct vmcs *, root_vmcs);

Perhaps introduce a CONFIG_INTEL_VMX for this? For example, KVM need not
be enabled if all one wants to do is use TDX to setup PCIe Link
Encryption. ...or were you expecting?

#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(...<other VMX users>...)