linux-kernel - Re: [PATCH v8 3/3] KVM: x86: Add "suspendsteal" cmdline to request host to add suspend duration in steal time

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Message-ID: <aKZr-4sZWFZXL7hP@google.com>
Date: Wed, 20 Aug 2025 17:44:43 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Suleiman Souhlal <suleiman@...gle.com>
Cc: 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>, Chao Gao <chao.gao@...el.com>, 
	David Woodhouse <dwmw2@...radead.org>, Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, Tzung-Bi Shih <tzungbi@...nel.org>, 
	John Stultz <jstultz@...gle.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	ssouhlal@...ebsd.org
Subject: Re: [PATCH v8 3/3] KVM: x86: Add "suspendsteal" cmdline to request
 host to add suspend duration in steal time

On Tue, Jul 22, 2025, Suleiman Souhlal wrote:
> Introduce a new command line parameter, "suspendsteal", enabling the
> guest to use MSR_KVM_SUSPEND_STEAL, which tells the host that it would
> like host suspend duration to be included in steal time.
> 
> Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>
> ---

And then if we reuse MSR_KVM_STEAL_TIME:

---
 Documentation/admin-guide/kernel-parameters.txt |  5 +++++
 arch/x86/kernel/kvm.c                           | 13 ++++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 747a55abf494..8e80094317c3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -7178,6 +7178,11 @@
 			improve throughput, but will also increase the
 			amount of memory reserved for use by the client.
 
+	suspendsteal
+			[X86,PV_OPS]
+			Enable requesting the host to include the duration the
+			host was suspended in steal time. Disabled by default.
+
 	suspend.pm_test_delay=
 			[SUSPEND]
 			Sets the number of seconds to remain in a suspend test
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 8ae750cde0c6..1eea3e82c85b 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -71,6 +71,7 @@ static DEFINE_PER_CPU_READ_MOSTLY(bool, async_pf_enabled);
 static DEFINE_PER_CPU_DECRYPTED(struct kvm_vcpu_pv_apf_data, apf_reason) __aligned(64);
 DEFINE_PER_CPU_DECRYPTED(struct kvm_steal_time, steal_time) __aligned(64) __visible;
 static int has_steal_clock = 0;
+static bool suspend_steal;
 
 static int has_guest_poll = 0;
 /*
@@ -320,6 +321,15 @@ static void __init paravirt_ops_setup(void)
 #endif
 }
 
+static int __init suspendsteal_setup(char *s)
+{
+	if (kvm_para_has_feature(KVM_FEATURE_SUSPEND_STEAL))
+		suspend_steal = true;
+
+	return 0;
+}
+early_param("suspendsteal", suspendsteal_setup);
+
 static void kvm_register_steal_time(void)
 {
 	int cpu = smp_processor_id();
@@ -328,7 +338,8 @@ static void kvm_register_steal_time(void)
 	if (!has_steal_clock)
 		return;
 
-	wrmsrq(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED));
+	wrmsrq(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED) |
+				   (suspend_steal ? KVM_STEAL_SUSPEND_TIME : 0));
 	pr_debug("stealtime: cpu %d, msr %llx\n", cpu,
 		(unsigned long long) slow_virt_to_phys(st));
 }
--