Paolo Bonzini <pbonzini@xxxxxxxxxx> 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
