Re: [PATCH v2] KVM: SVM: Set/clear SRSO's BP_SPEC_REDUCE on 0 <=> 1 VM count transitions

[Sean Christopherson <seanjc@google.com>]

On Tue, May 06, 2025, Yosry Ahmed wrote:
> On Mon, May 05, 2025 at 11:03:00AM -0700, Sean Christopherson wrote:
> > +static void svm_srso_vm_destroy(void)
> > +{
> > +	if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
> > +		return;
> > +
> > +	if (atomic_dec_return(&srso_nr_vms))
> > +		return;
> > +
> > +	guard(spinlock)(&srso_lock);
> > +
> > +	/*
> > +	 * Verify a new VM didn't come along, acquire the lock, and increment
> > +	 * the count before this task acquired the lock.
> > +	 */
> > +	if (atomic_read(&srso_nr_vms))
> > +		return;
> > +
> > +	on_each_cpu(svm_srso_clear_bp_spec_reduce, NULL, 1);
> 
> Just a passing-by comment. I get worried about sending IPIs while
> holding a spinlock because if someone ever tries to hold that spinlock
> with IRQs disabled, it may cause a deadlock.
> 
> This is not the case for this lock, but it's not obvious (at least to
> me) that holding it in a different code path that doesn't send IPIs with
> IRQs disabled could cause a problem.
> 
> You could add a comment, convert it to a mutex to make this scenario
> impossible,

Using a mutex doesn't make deadlock impossible, it's still perfectly legal to
disable IRQs while holding a mutex.

Similarly, I don't want to add a comment, because there is absolutely nothing
special/unique about this situation/lock.  E.g. KVM has tens of calls to
smp_call_function_many_cond() while holding a spinlock equivalent, in the form
of kvm_make_all_cpus_request() while holding mmu_lock.

smp_call_function_many_cond() already asserts that IRQs are disabled, so I have
zero concerns about this flow breaking in the future.

> or dismiss my comment as being too paranoid/ridiculous :)

I wouldn't say your thought process is too paranoid; when writing the code, I had
to pause and think to remember whether or not using on_each_cpu() while holding a
spinlock is allowed.  But I do think the conclusion is wrong :-)