x86/kvm/vmx: Defer TR reload after VM exit

Intel's VMX is daft and resets the hidden TSS limit register to 0x67
on VMX reload, and the 0x67 is not configurable.  KVM currently
reloads TR using the LTR instruction on every exit, but this is quite
slow because LTR is serializing.

The 0x67 limit is entirely harmless unless ioperm() is in use, so
defer the reload until a task using ioperm() is actually running.

Here's some poorly done benchmarking using kvm-unit-tests:

Before:

cpuid 1313
vmcall 1195
mov_from_cr8 11
mov_to_cr8 17
inl_from_pmtimer 6770
inl_from_qemu 6856
inl_from_kernel 2435
outl_to_kernel 1402

After:

cpuid 1291
vmcall 1181
mov_from_cr8 11
mov_to_cr8 16
inl_from_pmtimer 6457
inl_from_qemu 6209
inl_from_kernel 2339
outl_to_kernel 1391

Signed-off-by: Andy Lutomirski <luto@kernel.org>
[Force-reload TR in invalidate_tss_limit. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 48 insertions, 0 deletions

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 2e781bcc5e12..cb8f9149f6c8 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -205,6 +205,54 @@ static inline void native_load_tr_desc(void)
 	asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
 }
 
+static inline void force_reload_TR(void)
+{
+	struct desc_struct *d = get_cpu_gdt_table(smp_processor_id());
+	tss_desc tss;
+
+	memcpy(&tss, &d[GDT_ENTRY_TSS], sizeof(tss_desc));
+
+	/*
+	 * LTR requires an available TSS, and the TSS is currently
+	 * busy.  Make it be available so that LTR will work.
+	 */
+	tss.type = DESC_TSS;
+	write_gdt_entry(d, GDT_ENTRY_TSS, &tss, DESC_TSS);
+
+	load_TR_desc();
+}
+
+DECLARE_PER_CPU(bool, need_tr_refresh);
+
+static inline void refresh_TR(void)
+{
+	DEBUG_LOCKS_WARN_ON(preemptible());
+
+	if (unlikely(this_cpu_read(need_tr_refresh))) {
+		force_reload_TR();
+		this_cpu_write(need_tr_refresh, false);
+	}
+}
+
+/*
+ * If you do something evil that corrupts the cached TSS limit (I'm looking
+ * at you, VMX exits), call this function.
+ *
+ * The optimization here is that the TSS limit only matters for Linux if the
+ * IO bitmap is in use.  If the TSS limit gets forced to its minimum value,
+ * everything works except that IO bitmap will be ignored and all CPL 3 IO
+ * instructions will #GP, which is exactly what we want for normal tasks.
+ */
+static inline void invalidate_tss_limit(void)
+{
+	DEBUG_LOCKS_WARN_ON(preemptible());
+
+	if (unlikely(test_thread_flag(TIF_IO_BITMAP)))
+		force_reload_TR();
+	else
+		this_cpu_write(need_tr_refresh, true);
+}
+
 static inline void native_load_gdt(const struct desc_ptr *dtr)
 {
 	asm volatile("lgdt %0"::"m" (*dtr));