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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>
This commit is contained in:
Sean Christopherson 2019-01-25 07:41:02 -08:00 committed by Paolo Bonzini
parent fbda0fd31a
commit 5a8781607e
4 changed files with 13 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
arch/x86/kvm/vmx/nested.c
View File

@ -1979,17 +1979,6 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
if (vmx->nested.dirty_vmcs12 || vmx->nested.hv_evmcs)
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
*/
@ -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);

1
arch/x86/kvm/vmx/vmcs.h
View File

@ -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

13
arch/x86/kvm/vmx/vmx.c
View File

@ -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
);
}

1
arch/x86/kvm/vmx/vmx.h
View File

@ -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;