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SubjectRe: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V
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Radim Krčmář <rkrcmar@redhat.com> writes:

> 2018-03-12 15:19+0100, Vitaly Kuznetsov:
>>
>> That said I'd like to defer the question to KVM maintainers: Paolo,
>> Radim, what would you like me to do? Use STATIC_JUMP_IF_TRUE/FALSE as
>> they are, try to make them work for !HAVE_JUMP_LABEL and use them or
>> maybe we can commit the series as-is and have it as a future
>> optimization (e.g. when HAVE_JUMP_LABEL becomes mandatory)?
>
> Please take a look into making a macro that uses STATIC_JUMP_IF_FALSE or
> reads the value from provided static_key and does a test-jump, depending
> on HAVE_JUMP_LABEL.
> It doesn't need to be suited for general use, just something that moves
> the ugliness away from vmx_vcpu_run.
> (Although having it in jump_label.h would be great. I think the main
> obstacle is clobbering of flags.)
>

The other problem is that we actually have inline assembly and I'm not
sure how to use .macros from '#ifdef __ASSEMBLY__' sections there ...

anyway, I tried using the jump label magic and I ended up with the
following:

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 44b6efa7d54e..fb15ccf260fb 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -9932,10 +9932,26 @@ static void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, delta_tsc);
}

+#ifdef HAVE_JUMP_LABEL
+#define STATIC_CHECK_EVMCS_INUSE(label, key) \
+ ".Lstatic_evmcs:\n\t" \
+ ".byte 0xe9\n\t" \
+ ".long " #label " - .Lstatic_evmcs_after\n\t" \
+ ".Lstatic_evmcs_after:\n" \
+ ".pushsection __jump_table, \"aw\" \n\t" \
+ _ASM_ALIGN "\n\t" \
+ _ASM_PTR ".Lstatic_evmcs, " #label ", %c[" #key "] + 1 \n\t" \
+ ".popsection \n\t"
+#else
+#define STATIC_CHECK_EVMCS_INUSE(label, key) \
+ "cmpl $0, (%c[" #key "])\n\t" \
+ "je " #label "\n\t"
+#endif
+
static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
- unsigned long cr3, cr4, evmcs_rsp;
+ unsigned long cr3, cr4;

/* Record the guest's net vcpu time for enforced NMI injections. */
if (unlikely(!enable_vnmi &&
@@ -10002,9 +10018,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)

vmx->__launched = vmx->loaded_vmcs->launched;

- evmcs_rsp = static_branch_unlikely(&enable_evmcs) ?
- (unsigned long)&current_evmcs->host_rsp : 0;
-
asm(
/* Store host registers */
"push %%" _ASM_DX "; push %%" _ASM_BP ";"
@@ -10013,12 +10026,11 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
"cmp %%" _ASM_SP ", %c[host_rsp](%0) \n\t"
"je 1f \n\t"
"mov %%" _ASM_SP ", %c[host_rsp](%0) \n\t"
- /* Avoid VMWRITE when Enlightened VMCS is in use */
- "test %%" _ASM_SI ", %%" _ASM_SI " \n\t"
- "jz 2f \n\t"
- "mov %%" _ASM_SP ", (%%" _ASM_SI ") \n\t"
+ /* Avoid VMWRITE to HOST_SP when Enlightened VMCS is in use */
+ STATIC_CHECK_EVMCS_INUSE(.Lvmwrite_sp, enable_evmcs)
+ "mov %%" _ASM_SP ", %c[evmcs_hrsp](%2) \n\t"
"jmp 1f \n\t"
- "2: \n\t"
+ ".Lvmwrite_sp: \n\t"
__ex(ASM_VMX_VMWRITE_RSP_RDX) "\n\t"
"1: \n\t"
/* Reload cr2 if changed */
@@ -10096,10 +10108,12 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
".global vmx_return \n\t"
"vmx_return: " _ASM_PTR " 2b \n\t"
".popsection"
- : : "c"(vmx), "d"((unsigned long)HOST_RSP), "S"(evmcs_rsp),
+ : : "c"(vmx), "d"((unsigned long)HOST_RSP), "S"(current_evmcs),
+ [enable_evmcs]"i"(&enable_evmcs),
[launched]"i"(offsetof(struct vcpu_vmx, __launched)),
[fail]"i"(offsetof(struct vcpu_vmx, fail)),
[host_rsp]"i"(offsetof(struct vcpu_vmx, host_rsp)),
+ [evmcs_hrsp]"i"(offsetof(struct hv_enlightened_vmcs, host_rsp)),
[rax]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RAX])),
[rbx]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RBX])),
[rcx]"i"(offsetof(struct vcpu_vmx, vcpu.arch.regs[VCPU_REGS_RCX])),
What I particularly dislike is that we now depend on jump labels
internals. Generalizing this hack doesn't seem practical as
non-HAVE_JUMP_LABEL path cloobbers FLAGS and requiring users to know
that is cumbersome...

I'd say 'too ugly' but I can continue investigating if there're fresh ideas.

--
Vitaly

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