linux-kernel - Re: x86/bugs: KVM: Add support for SRSO_MSR

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Message-ID: <20250501081918.GAaBMuhq6Qaa0C_xk_@fat_crate.local>
Date: Thu, 1 May 2025 10:19:18 +0200
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>,
	Patrick Bellasi <derkling@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Patrick Bellasi <derkling@...bug.net>,
	Brendan Jackman <jackmanb@...gle.com>,
	David Kaplan <David.Kaplan@....com>,
	Michael Larabel <Michael@...haellarabel.com>
Subject: Re: x86/bugs: KVM: Add support for SRSO_MSR_FIX, back for moar

On Wed, Apr 30, 2025 at 04:33:19PM -0700, Sean Christopherson wrote:
> Eww.  That's quite painful, and completely disallowing enable_virt_on_load is
> undesirable, e.g. for use cases where the host is (almost) exclusively running
> VMs.

I wanted to stay generic... :-)

> Best idea I have is to throw in the towel on getting fancy, and just maintain a
> dedicated count in SVM.
> 
> Alternatively, we could plumb an arch hook into kvm_create_vm() and kvm_destroy_vm()
> that's called when KVM adds/deletes a VM from vm_list, and key off vm_list being
> empty.  But that adds a lot of boilerplate just to avoid a mutex+count.

FWIW, that was Tom's idea.

> +static void svm_srso_add_remove_vm(int count)
> +{
> +	bool set;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
> +		return;
> +
> +	guard(mutex)(&srso_lock);
> +
> +	set = !srso_nr_vms;
> +	srso_nr_vms += count;
> +
> +	WARN_ON_ONCE(srso_nr_vms < 0);
> +	if (!set && srso_nr_vms)
> +		return;

So instead of doing this "by-foot", I would've used any of those
atomic_inc_return() and atomic_dec_and_test() and act upon the value when it
becomes 0 or !0 instead of passing 1 and -1. Because the count is kinda
implicit...

But yeah, not a biggie - that solves the issue too.

Thanks!

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette