x86/mm, KVM: Teach KVM's VMX code that CR3 isn't a constant

When PCID is enabled, CR3's PCID bits can change during context
switches, so KVM won't be able to treat CR3 as a per-mm constant any
more.

I structured this like the existing CR4 handling.  Under ordinary
circumstances (PCID disabled or if the current PCID and the value
that's already in the VMCS match), then we won't do an extra VMCS
write, and we'll never do an extra direct CR3 read.  The overhead
should be minimal.

I disallowed using the new helper in non-atomic context because
PCID support will cause CR3 to stop being constant in non-atomic
process context.

(Frankly, it also scares me a bit that KVM ever treated CR3 as
constant, but it looks like it was okay before.)

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 187c39470a0b..f20d7ea47095 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -266,4 +266,23 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
 	return __pkru_allows_pkey(vma_pkey(vma), write);
 }
 
+
+/*
+ * This can be used from process context to figure out what the value of
+ * CR3 is without needing to do a (slow) read_cr3().
+ *
+ * It's intended to be used for code like KVM that sneakily changes CR3
+ * and needs to restore it.  It needs to be used very carefully.
+ */
+static inline unsigned long __get_current_cr3_fast(void)
+{
+	unsigned long cr3 = __pa(this_cpu_read(cpu_tlbstate.loaded_mm)->pgd);
+
+	/* For now, be very restrictive about when this can be called. */
+	VM_WARN_ON(in_nmi() || !in_atomic());
+
+	VM_BUG_ON(cr3 != read_cr3());
+	return cr3;
+}
+
 #endif /* _ASM_X86_MMU_CONTEXT_H */