linux-kernel - [PATCH v2] KVM: x86/xen: ignore the VCPU_SSHOTTMR

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Date:   Wed,  4 Oct 2023 17:46:28 +0000
From:   Paul Durrant <paul@....org>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Paul Durrant <pdurrant@...zon.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        David Woodhouse <dwmw2@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        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>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: [PATCH v2] KVM: x86/xen: ignore the VCPU_SSHOTTMR_future flag

From: Paul Durrant <pdurrant@...zon.com>

Upstream Xen now ignores this flag [1], since the only guest kernel ever to
use it was buggy. By ignoring the flag the guest will always get a callback
if it sets a negative timeout which upstream Xen has determined not to
cause problems for any guest setting the flag.

[1] https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=19c6cbd909

Signed-off-by: Paul Durrant <pdurrant@...zon.com>
Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>
---
Cc: David Woodhouse <dwmw2@...radead.org>
Cc: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: x86@...nel.org

v2:
 - Amend the patch subject line
---
 arch/x86/kvm/xen.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 40edf4d1974c..8f1d46df0f3b 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1374,12 +1374,8 @@ static bool kvm_xen_hcall_vcpu_op(struct kvm_vcpu *vcpu, bool longmode, int cmd,
 			return true;
 		}
 
+		/* A delta <= 0 results in an immediate callback, which is what we want */
 		delta = oneshot.timeout_abs_ns - get_kvmclock_ns(vcpu->kvm);
-		if ((oneshot.flags & VCPU_SSHOTTMR_future) && delta < 0) {
-			*r = -ETIME;
-			return true;
-		}
-
 		kvm_xen_start_timer(vcpu, oneshot.timeout_abs_ns, delta);
 		*r = 0;
 		return true;
-- 
2.39.2