From: Christian Borntraeger <borntraeger@xxxxxxxxxx>
On some cpus the overhead for virtualization instructions is in the same
range as a system call. Having to call multiple ioctls to get set registers
will make userspace handled exits more expensive than necessary.
Lets provide two sections in kvm_run to have a shared save area for
guest registers.
1. the first section is read-only, to handle registers that have side-effects
2. the second section is read/write, e.g. for general purpose registers.
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
Documentation/virtual/kvm/api.txt | 16 ++++++++++++++++
arch/ia64/include/asm/kvm.h | 7 +++++++
arch/powerpc/include/asm/kvm.h | 7 +++++++
arch/s390/include/asm/kvm.h | 6 ++++++
arch/x86/include/asm/kvm.h | 7 +++++++
include/linux/kvm.h | 13 +++++++++++++
6 files changed, 56 insertions(+)
Index: b/Documentation/virtual/kvm/api.txt
===================================================================
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -1652,7 +1652,23 @@ developer registration required to acces
/* Fix the size of the union. */
char padding[256];
};
+ /* Here are two fields that allow to access often used registers
+ * directly, to avoid the overhead of the ioctl system call */
+ union {
+ /* registers which can be only read */
+ struct sync_ro_regs sync_ro_regs;
+ char padding[1024];
+ };
+ union {
+ /* read/write guest registers */
+ struct sync_rw_regs sync_rw_regs;
+ char padding[1024];
+ };
};
+These fields allow userspace to access certain guest registers without
+having to call SET/GET_*REGS. Thus we can avoid some system call
+overhead if userspace has to handle the exit. (only available if
+KVM_CAP_SYNC_REGS is set). The ioctls will still work.
6. Capabilities that can be enabled
Index: b/arch/ia64/include/asm/kvm.h
===================================================================
--- a/arch/ia64/include/asm/kvm.h
+++ b/arch/ia64/include/asm/kvm.h
@@ -261,4 +261,11 @@ struct kvm_debug_exit_arch {
struct kvm_guest_debug_arch {
};
+/* definition of registers in kvm_run */
+struct sync_ro_regs {
+};
+
+struct sync_rw_regs {
+};
+
#endif
Index: b/arch/powerpc/include/asm/kvm.h
===================================================================
--- a/arch/powerpc/include/asm/kvm.h
+++ b/arch/powerpc/include/asm/kvm.h
@@ -265,6 +265,13 @@ struct kvm_debug_exit_arch {
struct kvm_guest_debug_arch {
};
+/* definition of registers in kvm_run */
+struct sync_ro_regs {
+};
+
+struct sync_rw_regs {
+};
+
#define KVM_REG_MASK 0x001f
#define KVM_REG_EXT_MASK 0xffe0
#define KVM_REG_GPR 0x0000
Index: b/arch/s390/include/asm/kvm.h
===================================================================
--- a/arch/s390/include/asm/kvm.h
+++ b/arch/s390/include/asm/kvm.h
@@ -41,4 +41,10 @@ struct kvm_debug_exit_arch {
struct kvm_guest_debug_arch {
};
+/* definition of registers in kvm_run */
+struct sync_ro_regs {
+};
+
+struct sync_rw_regs {
+};
#endif
Index: b/arch/x86/include/asm/kvm.h
===================================================================
--- a/arch/x86/include/asm/kvm.h
+++ b/arch/x86/include/asm/kvm.h
@@ -321,4 +321,11 @@ struct kvm_xcrs {
__u64 padding[16];
};
+/* definition of registers in kvm_run */
+struct sync_ro_regs {
+};
+
+struct sync_rw_regs {
+};
+
#endif /* _ASM_X86_KVM_H */
Index: b/include/linux/kvm.h
===================================================================
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -273,6 +273,18 @@ struct kvm_run {
/* Fix the size of the union. */
char padding[256];
};
+ /* Here are two fields that allow to access often used registers
+ * directly, to avoid the overhead of the ioctl system call */
+ union {
+ /* registers which can be only read */
+ struct sync_ro_regs sync_ro_regs;
+ char padding[1024];
+ };
+ union {
+ /* read/write guest registers */
+ struct sync_rw_regs sync_rw_regs;
+ char padding[1024];
+ };
};
/* for KVM_REGISTER_COALESCED_MMIO / KVM_UNREGISTER_COALESCED_MMIO */
@@ -557,6 +569,7 @@ struct kvm_ppc_pvinfo {
#define KVM_CAP_MAX_VCPUS 66 /* returns max vcpus per vm */
#define KVM_CAP_PPC_PAPR 68
#define KVM_CAP_S390_GMAP 71
+#define KVM_CAP_SYNC_REGS 72
#ifdef KVM_CAP_IRQ_ROUTING
--
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