linux-kernel - Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

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Date: Tue, 06 Feb 2024 10:51:28 -0800
From: David Woodhouse <dwmw2@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm <kvm@...r.kernel.org>, Paul Durrant <paul@....org>, Paolo Bonzini
 <pbonzini@...hat.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, "H. Peter Anvin"
 <hpa@...or.com>,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

On Tue, 2024-02-06 at 10:41 -0800, Sean Christopherson wrote:
> 
> This has an obvious-in-hindsight recursive deadlock bug.  If KVM actually needs
> to inject a timer IRQ, and the fast path fails, i.e. the gpc is invalid,
> kvm_xen_set_evtchn() will attempt to acquire xen.xen_lock, which is already held

Hm, right. In fact, kvm_xen_set_evtchn() shouldn't actually *need* the
xen_lock in an ideal world; it's only taking it in order to work around
the fact that the gfn_to_pfn_cache doesn't have its *own* self-
sufficient locking. I have patches for that...

I think the *simplest* of the "patches for that" approaches is just to
use the gpc->refresh_lock to cover all activate, refresh and deactivate
calls. I was waiting for Paul's series to land before sending that one,
but I'll work on it today, and double-check my belief that we can then
just drop xen_lock from kvm_xen_set_evtchn().

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