From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, x86@kernel.org,
"Radim Krčmář" <rkrcmar@redhat.com>,
"K. Y. Srinivasan" <kys@microsoft.com>,
"Haiyang Zhang" <haiyangz@microsoft.com>,
"Stephen Hemminger" <sthemmin@microsoft.com>,
"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
"Mohammed Gamal" <mmorsy@redhat.com>,
"Cathy Avery" <cavery@redhat.com>, "Bandan Das" <bsd@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V
Date: Thu, 15 Mar 2018 16:19:50 +0100 [thread overview]
Message-ID: <87zi399xih.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <2747cc75-b549-61bb-9c1b-0f554a49b536@redhat.com> (Paolo Bonzini's message of "Wed, 14 Mar 2018 15:53:32 +0100")
Paolo Bonzini <pbonzini@redhat.com> writes:
> On 09/03/2018 15:02, Vitaly Kuznetsov wrote:
>> Enlightened VMCS is just a structure in memory, the main benefit
>> besides avoiding somewhat slower VMREAD/VMWRITE is using clean field
>> mask: we tell the underlying hypervisor which fields were modified
>> since VMEXIT so there's no need to inspect them all.
>>
>> Tight CPUID loop test shows significant speedup:
>> Before: 18890 cycles
>> After: 8304 cycles
>>
>> Static key is being used to avoid performance penalty for non-Hyper-V
>> deployments. Tests show we add around 3 (three) CPU cycles on each
>> VMEXIT (1077.5 cycles before, 1080.7 cycles after for the same CPUID
>> loop on bare metal). We can probably avoid one test/jmp in vmx_vcpu_run()
>> but I don't see a clean way to use static key in assembly.
>
> If you want to live dangerously, you can use text_poke_early to change
> the vmwrite to mov. It's just a single instruction, so it's probably
> not too hard.
It is not:
+#if IS_ENABLED(CONFIG_HYPERV) && defined(CONFIG_X86_64)
+
+/* Luckily, both original and new instructions are of the same length */
+#define EVMCS_RSP_OPCODE_LEN 3
+static evmcs_patch_vmx_cpu_run(void)
+{
+ u8 *addr;
+ u8 opcode_old[] = {0x0f, 0x79, 0xd4}; // vmwrite rsp, rdx
+ u8 opcode_new[] = {0x48, 0x89, 0x26}; // mov rsp, (rsi)
+
+ /*
+ * What we're searching for MUST be present in vmx_cpu_run().
+ * We replace the first occurance only.
+ */
+ for (addr = (u8 *)vmx_vcpu_run; ; addr++) {
+ if (!memcmp(addr, opcode_old, EVMCS_RSP_OPCODE_LEN)) {
+ /*
+ * vmx_vcpu_run is not currently running on other CPUs but
+ * using text_poke_early() would require us to do manual
+ * RW remapping of the area.
+ */
+ text_poke(addr, opcode_new, EVMCS_RSP_OPCODE_LEN);
+ break;
+ }
+ }
+}
+#endif
+
text_poke() also needs to be exported.
This works. But hell, this is a crude hack :-) Not sure if there's a
cleaner way to find what needs to be patched without something like jump
label table ...
--
Vitaly
next prev parent reply other threads:[~2018-03-15 15:19 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-09 14:02 [PATCH v3 0/7] Enlightened VMCS support for KVM on Hyper-V Vitaly Kuznetsov
2018-03-09 14:02 ` [PATCH v3 1/7] x86/hyper-v: move hyperv.h out of uapi Vitaly Kuznetsov
2018-03-13 22:46 ` Michael Kelley (EOSG)
2018-03-14 9:35 ` Vitaly Kuznetsov
2018-03-14 16:13 ` Christoph Hellwig
2018-03-14 16:42 ` Joshua R. Poulson
2018-03-15 7:31 ` Christoph Hellwig
2018-03-09 14:02 ` [PATCH v3 2/7] x86/hyper-v: move definitions from TLFS to hyperv-tlfs.h Vitaly Kuznetsov
2018-03-13 22:51 ` Michael Kelley (EOSG)
2018-03-09 14:02 ` [PATCH v3 3/7] x86/kvm: rename HV_X64_MSR_APIC_ASSIST_PAGE to HV_X64_MSR_VP_ASSIST_PAGE Vitaly Kuznetsov
2018-03-09 14:02 ` [PATCH v3 4/7] x86/hyper-v: allocate and use Virtual Processor Assist Pages Vitaly Kuznetsov
2018-03-13 23:08 ` Michael Kelley (EOSG)
2018-03-14 15:15 ` Thomas Gleixner
2018-03-15 10:10 ` Vitaly Kuznetsov
2018-03-15 11:45 ` Thomas Gleixner
2018-03-15 13:48 ` Peter Zijlstra
2018-03-15 13:57 ` Thomas Gleixner
2018-03-09 14:02 ` [PATCH v3 5/7] x86/hyper-v: define struct hv_enlightened_vmcs and clean field bits Vitaly Kuznetsov
2018-03-13 23:09 ` Michael Kelley (EOSG)
2018-03-09 14:02 ` [PATCH v3 6/7] x86/hyper-v: detect nested features Vitaly Kuznetsov
2018-03-13 23:11 ` Michael Kelley (EOSG)
2018-03-09 14:02 ` [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V Vitaly Kuznetsov
2018-03-09 14:08 ` Thomas Gleixner
2018-03-12 14:19 ` Vitaly Kuznetsov
2018-03-13 19:12 ` Radim Krčmář
2018-03-14 17:20 ` Vitaly Kuznetsov
2018-03-14 14:54 ` Paolo Bonzini
2018-03-14 15:19 ` Thomas Gleixner
2018-03-14 17:22 ` Vitaly Kuznetsov
2018-03-14 19:59 ` Thomas Gleixner
2018-03-14 20:06 ` Peter Zijlstra
2018-03-14 14:53 ` Paolo Bonzini
2018-03-15 9:56 ` Vitaly Kuznetsov
2018-03-15 11:01 ` Paolo Bonzini
2018-03-15 15:19 ` Vitaly Kuznetsov [this message]
2018-03-15 17:02 ` Radim Krčmář
2018-03-15 17:28 ` Radim Krčmář
2018-03-15 18:04 ` Vitaly Kuznetsov
2018-03-15 19:28 ` Thomas Gleixner
2018-03-15 19:43 ` Radim Krčmář
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