powerpc/mm/radix: Workaround prefetch issue with KVM

There's a somewhat architectural issue with Radix MMU and KVM.

When coming out of a guest with AIL (Alternate Interrupt Location, ie,
MMU enabled), we start executing hypervisor code with the PID register
still containing whatever the guest has been using.

The problem is that the CPU can (and will) then start prefetching or
speculatively load from whatever host context has that same PID (if
any), thus bringing translations for that context into the TLB, which
Linux doesn't know about.

This can cause stale translations and subsequent crashes.

Fixing this in a way that is neither racy nor a huge performance
impact is difficult. We could just make the host invalidations always
use broadcast forms but that would hurt single threaded programs for
example.

We chose to fix it instead by partitioning the PID space between guest
and host. This is possible because today Linux only use 19 out of the
20 bits of PID space, so existing guests will work if we make the host
use the top half of the 20 bits space.

We additionally add support for a property to indicate to Linux the
size of the PID register which will be useful if we eventually have
processors with a larger PID space available.

There is still an issue with malicious guests purposefully setting the
PID register to a value in the hosts PID range. Hopefully future HW
can prevent that, but in the meantime, we handle it with a pair of
kludges:

 - On the way out of a guest, before we clear the current VCPU in the
   PACA, we check the PID and if it's outside of the permitted range
   we flush the TLB for that PID.

 - When context switching, if the mm is "new" on that CPU (the
   corresponding bit was set for the first time in the mm cpumask), we
   check if any sibling thread is in KVM (has a non-NULL VCPU pointer
   in the PACA). If that is the case, we also flush the PID for that
   CPU (core).

This second part is needed to handle the case where a process is
migrated (or starts a new pthread) on a sibling thread of the CPU
coming out of KVM, as there's a window where stale translations can
exist before we detect it and flush them out.

A future optimization could be added by keeping track of whether the
PID has ever been used and avoid doing that for completely fresh PIDs.
We could similarily mark PIDs that have been the subject of a global
invalidation as "fresh". But for now this will do.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Rework the asm to build with CONFIG_PPC_RADIX_MMU=n, drop
      unneeded include of kvm_book3s_asm.h]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 43 insertions, 2 deletions

diff --git a/arch/powerpc/mm/tlb-radix.c b/arch/powerpc/mm/tlb-radix.c
index 744e0164ecf5..16ae1bbe13f0 100644
--- a/arch/powerpc/mm/tlb-radix.c
+++ b/arch/powerpc/mm/tlb-radix.c
@@ -12,12 +12,12 @@
 #include <linux/mm.h>
 #include <linux/hugetlb.h>
 #include <linux/memblock.h>
-#include <asm/ppc-opcode.h>
 
+#include <asm/ppc-opcode.h>
 #include <asm/tlb.h>
 #include <asm/tlbflush.h>
 #include <asm/trace.h>
-
+#include <asm/cputhreads.h>
 
 #define RIC_FLUSH_TLB 0
 #define RIC_FLUSH_PWC 1
@@ -454,3 +454,44 @@ void radix__flush_tlb_pte_p9_dd1(unsigned long old_pte, struct mm_struct *mm,
 	else
 		radix__flush_tlb_page_psize(mm, address, mmu_virtual_psize);
 }
+
+#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
+extern void radix_kvm_prefetch_workaround(struct mm_struct *mm)
+{
+	unsigned int pid = mm->context.id;
+
+	if (unlikely(pid == MMU_NO_CONTEXT))
+		return;
+
+	/*
+	 * If this context hasn't run on that CPU before and KVM is
+	 * around, there's a slim chance that the guest on another
+	 * CPU just brought in obsolete translation into the TLB of
+	 * this CPU due to a bad prefetch using the guest PID on
+	 * the way into the hypervisor.
+	 *
+	 * We work around this here. If KVM is possible, we check if
+	 * any sibling thread is in KVM. If it is, the window may exist
+	 * and thus we flush that PID from the core.
+	 *
+	 * A potential future improvement would be to mark which PIDs
+	 * have never been used on the system and avoid it if the PID
+	 * is new and the process has no other cpumask bit set.
+	 */
+	if (cpu_has_feature(CPU_FTR_HVMODE) && radix_enabled()) {
+		int cpu = smp_processor_id();
+		int sib = cpu_first_thread_sibling(cpu);
+		bool flush = false;
+
+		for (; sib <= cpu_last_thread_sibling(cpu) && !flush; sib++) {
+			if (sib == cpu)
+				continue;
+			if (paca[sib].kvm_hstate.kvm_vcpu)
+				flush = true;
+		}
+		if (flush)
+			_tlbiel_pid(pid, RIC_FLUSH_ALL);
+	}
+}
+EXPORT_SYMBOL_GPL(radix_kvm_prefetch_workaround);
+#endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */