KVM: nVMX: Cache host_rsp on a per-VMCS basis

Currently, host_rsp is cached on a per-vCPU basis, i.e. it's stored in
struct vcpu_vmx.  In non-nested usage the caching is for all intents
and purposes 100% effective, e.g. only the first VMLAUNCH needs to
synchronize VMCS.HOST_RSP since the call stack to vmx_vcpu_run() is
identical each and every time.  But when running a nested guest, KVM
must invalidate the cache when switching the current VMCS as it can't
guarantee the new VMCS has the same HOST_RSP as the previous VMCS.  In
other words, the cache loses almost all of its efficacy when running a
nested VM.

Move host_rsp to struct vmcs_host_state, which is per-VMCS, so that it
is cached on a per-VMCS basis and restores its 100% hit rate when
nested VMs are in play.

Note that the host_rsp cache for vmcs02 essentially "breaks" when
nested early checks are enabled as nested_vmx_check_vmentry_hw() will
see a different RSP at the time of its VM-Enter.  While it's possible
to avoid even that VMCS.HOST_RSP synchronization, e.g. by employing a
dedicated VM-Exit stack, there is little motivation for doing so as
the overhead of two VMWRITEs (~55 cycles) is dwarfed by the overhead
of the extra VMX transition (600+ cycles) and is a proverbial drop in
the ocean relative to the total cost of a nested transtion (10s of
thousands of cycles).

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 23a2c1b..0e67649 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -1980,17 +1980,6 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
                prepare_vmcs02_early_full(vmx, vmcs12);
 
        /*
-        * HOST_RSP is normally set correctly in vmx_vcpu_run() just before
-        * entry, but only if the current (host) sp changed from the value
-        * we wrote last (vmx->host_rsp).  This cache is no longer relevant
-        * if we switch vmcs, and rather than hold a separate cache per vmcs,
-        * here we just force the write to happen on entry.  host_rsp will
-        * also be written unconditionally by nested_vmx_check_vmentry_hw()
-        * if we are doing early consistency checks via hardware.
-        */
-       vmx->host_rsp = 0;
-
-       /*
         * PIN CONTROLS
         */
        exec_control = vmcs12->pin_based_vm_exec_control;
@@ -2754,8 +2743,11 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
 
        asm(
                "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* temporarily adjust RSP for CALL */
+               "cmp %%" _ASM_SP ", %c[host_state_rsp](%[loaded_vmcs]) \n\t"
+               "je 1f \n\t"
                __ex("vmwrite %%" _ASM_SP ", %[HOST_RSP]") "\n\t"
-               "mov %%" _ASM_SP ", %c[host_rsp](%% " _ASM_CX ")\n\t"
+               "mov %%" _ASM_SP ", %c[host_state_rsp](%[loaded_vmcs]) \n\t"
+               "1: \n\t"
                "add $%c[wordsize], %%" _ASM_SP "\n\t" /* un-adjust RSP */
 
                /* Check if vmlaunch or vmresume is needed */
@@ -2771,11 +2763,10 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
 
                CC_SET(be)
              : ASM_CALL_CONSTRAINT, CC_OUT(be) (vm_fail)
-             : "c"(vmx),
-               [HOST_RSP]"r"((unsigned long)HOST_RSP),
+             : [HOST_RSP]"r"((unsigned long)HOST_RSP),
                [loaded_vmcs]"r"(vmx->loaded_vmcs),
                [launched]"i"(offsetof(struct loaded_vmcs, launched)),
-               [host_rsp]"i"(offsetof(struct vcpu_vmx, host_rsp)),
+               [host_state_rsp]"i"(offsetof(struct loaded_vmcs, host_state.rsp)),
                [wordsize]"i"(sizeof(ulong))
              : "cc", "memory"
        );
@@ -3912,9 +3903,6 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
                vmx_flush_tlb(vcpu, true);
        }
 
-       /* This is needed for same reason as it was needed in prepare_vmcs02 */
-       vmx->host_rsp = 0;
-
        /* Unpin physical memory we referred to in vmcs02 */
        if (vmx->nested.apic_access_page) {
                kvm_release_page_dirty(vmx->nested.apic_access_page);

diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
index 6def3ba..cb6079f 100644 (file)
--- a/arch/x86/kvm/vmx/vmcs.h
+++ b/arch/x86/kvm/vmx/vmcs.h
@@ -34,6 +34,7 @@ struct vmcs_host_state {
        unsigned long cr4;      /* May not match real cr4 */
        unsigned long gs_base;
        unsigned long fs_base;
+       unsigned long rsp;
 
        u16           fs_sel, gs_sel, ldt_sel;
 #ifdef CONFIG_X86_64

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e9d81cd..12bb61e 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6381,9 +6381,9 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
                "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* placeholder for guest RCX */
                "push %%" _ASM_CX " \n\t"
                "sub $%c[wordsize], %%" _ASM_SP "\n\t" /* temporarily adjust RSP for CALL */
-               "cmp %%" _ASM_SP ", %c[host_rsp](%%" _ASM_CX ") \n\t"
+               "cmp %%" _ASM_SP ", (%%" _ASM_DI ") \n\t"
                "je 1f \n\t"
-               "mov %%" _ASM_SP ", %c[host_rsp](%%" _ASM_CX ") \n\t"
+               "mov %%" _ASM_SP ", (%%" _ASM_DI ") \n\t"
                /* Avoid VMWRITE when Enlightened VMCS is in use */
                "test %%" _ASM_SI ", %%" _ASM_SI " \n\t"
                "jz 2f \n\t"
@@ -6482,11 +6482,10 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
                "xor %%edi, %%edi \n\t"
                "xor %%ebp, %%ebp \n\t"
                "pop  %%" _ASM_BP " \n\t"
-             : ASM_CALL_CONSTRAINT, "=S"((int){0})
-             : "c"(vmx), "S"(evmcs_rsp),
+             : ASM_CALL_CONSTRAINT, "=D"((int){0}), "=S"((int){0})
+             : "c"(vmx), "D"(&vmx->loaded_vmcs->host_state.rsp), "S"(evmcs_rsp),
                [launched]"i"(offsetof(struct vcpu_vmx, __launched)),
                [fail]"i"(offsetof(struct vcpu_vmx, fail)),
-               [host_rsp]"i"(offsetof(struct vcpu_vmx, host_rsp)),
                [HOST_RSP]"i"(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])),
@@ -6509,10 +6508,10 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx)
                [wordsize]"i"(sizeof(ulong))
              : "cc", "memory"
 #ifdef CONFIG_X86_64
-               , "rax", "rbx", "rdx", "rdi"
+               , "rax", "rbx", "rdx"
                , "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
 #else
-               , "eax", "ebx", "edx", "edi"
+               , "eax", "ebx", "edx"
 #endif
              );
 }

diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 9932895..8e203b7 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -175,7 +175,6 @@ struct nested_vmx {
 
 struct vcpu_vmx {
        struct kvm_vcpu       vcpu;
-       unsigned long         host_rsp;
        u8                    fail;
        u8                    msr_bitmap_mode;
        u32                   exit_intr_info;