linux-kernel - [PATCH] x86/hyper-v: implement EOI assist

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Date:   Wed,  3 Apr 2019 19:03:09 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     linux-hyperv@...r.kernel.org
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Long Li <longli@...rosoft.com>,
        Simon Xiao <sixiao@...rosoft.com>
Subject: [PATCH] x86/hyper-v: implement EOI assist

Hyper-V TLFS suggests an optimization to avoid imminent VMExit on EOI:
"The OS performs an EOI by atomically writing zero to the EOI Assist field
of the virtual VP assist page and checking whether the "No EOI required"
field was previously zero. If it was, the OS must write to the
HV_X64_APIC_EOI MSR thereby triggering an intercept into the hypervisor."

Implement the optimization in Linux.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
 arch/x86/hyperv/hv_apic.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
index 8eb6fbee8e13..5c056b8aebef 100644
--- a/arch/x86/hyperv/hv_apic.c
+++ b/arch/x86/hyperv/hv_apic.c
@@ -86,6 +86,11 @@ static void hv_apic_write(u32 reg, u32 val)
 
 static void hv_apic_eoi_write(u32 reg, u32 val)
 {
+	struct hv_vp_assist_page *hvp = hv_vp_assist_page[smp_processor_id()];
+
+	if (hvp && (xchg(&hvp->apic_assist, 0) & 0x1))
+		return;
+
 	wrmsr(HV_X64_MSR_EOI, val, 0);
 }
 
-- 
2.20.1