KVM: PPC: Book3S PR: Disable SCV when AIL could be disabled

PR KVM does not support running with AIL enabled, and SCV does is not
supported with AIL disabled. Fix this by ensuring the SCV facility is
disabled with FSCR while a CPU could be running with AIL=0.

The PowerNV host supports disabling AIL on a per-CPU basis, so SCV just
needs to be disabled when a vCPU is being run.

The pSeries machine can only switch AIL on a system-wide basis, so it
must disable SCV support at boot if the configuration can potentially
run a PR KVM guest.

Also ensure a the FSCR[SCV] bit can not be enabled when emulating
mtFSCR for the guest.

SCV is not emulated for the PR guest at the moment, this just fixes the
host crashes.

Alternatives considered and rejected:
- SCV support can not be disabled by PR KVM after boot, because it is
  advertised to userspace with HWCAP.
- AIL can not be disabled on a per-CPU basis. At least when running on
  pseries it is a per-LPAR setting.
- Support for real-mode SCV vectors will not be added because they are
  at 0x17000 so making such a large fixed head space causes immediate
  value limits to be exceeded, requiring a lot rework and more code.
- Disabling SCV for any PR KVM possible kernel will cause a slowdown
  when not using PR KVM.
- A boot time option to disable SCV to use PR KVM is user-hostile.
- System call instruction emulation for SCV facility unavailable
  instructions is too complex and old emulation code was subtly broken
  and removed.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Link: https://lore.kernel.org/r/20220222064727.2314380-2-npiggin@gmail.com

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
index 18e58085447c..ddd88179110a 100644
--- a/arch/powerpc/kvm/Kconfig
+++ b/arch/powerpc/kvm/Kconfig
@@ -112,12 +112,21 @@ config KVM_BOOK3S_64_PR
 	  guest in user mode (problem state) and emulating all
 	  privileged instructions and registers.
 
+	  This is only available for hash MMU mode and only supports
+	  guests that use hash MMU mode.
+
 	  This is not as fast as using hypervisor mode, but works on
 	  machines where hypervisor mode is not available or not usable,
 	  and can emulate processors that are different from the host
 	  processor, including emulating 32-bit processors on a 64-bit
 	  host.
 
+	  Selecting this option will cause the SCV facility to be
+	  disabled when the kernel is booted on the pseries platform in
+	  hash MMU mode (regardless of PR VMs running). When any PR VMs
+	  are running, "AIL" mode is disabled which may slow interrupts
+	  and system calls on the host.
+
 config KVM_BOOK3S_HV_EXIT_TIMING
 	bool "Detailed timing for hypervisor real-mode code"
 	depends on KVM_BOOK3S_HV_POSSIBLE && DEBUG_FS