Re: [PATCH v2 7/7] arm64: nofpsmid: Handle TIF_FOREIGN_FPSTATE flag cleanly
From: Suzuki Kuruppassery Poulose
Date: Wed Dec 18 2019 - 07:00:32 EST
On 18/12/2019 11:56, Marc Zyngier wrote:
On 2019-12-18 11:42, Suzuki Kuruppassery Poulose wrote:
Hi Marc,
On 17/12/2019 19:05, Marc Zyngier wrote:
Hi Suzuki,
On 2019-12-17 18:34, Suzuki K Poulose wrote:
We detect the absence of FP/SIMD after an incapable CPU is brought up,
and by then we have kernel threads running already with
TIF_FOREIGN_FPSTATE set
which could be set for early userspace applications (e.g, modprobe
triggered
from initramfs) and init. This could cause the applications to loop
forever in
do_nofity_resume() as we never clear the TIF flag, once we now know
that
we don't support FP.
Fix this by making sure that we clear the TIF_FOREIGN_FPSTATE flag
for tasks which may have them set, as we would have done in the normal
case, but avoiding touching the hardware state (since we don't
support any).
Also to make sure we handle the cases seemlessly we categorise the
helper functions to two :
Â1) Helpers for common core code, which calls into take appropriate
ÂÂÂ actions without knowing the current FPSIMD state of the CPU/task.
ÂÂÂ e.g fpsimd_restore_current_state(), fpsimd_flush_task_state(),
ÂÂÂÂÂÂÂ fpsimd_save_and_flush_cpu_state().
ÂÂÂ We bail out early for these functions, taking any appropriate
actions
ÂÂÂ (e.g, clearing the TIF flag) where necessary to hide the handling
ÂÂÂ from core code.
Â2) Helpers used when the presence of FP/SIMD is apparent.
ÂÂÂ i.e, save/restore the FP/SIMD register state, modify the CPU/task
ÂÂÂ FP/SIMD state.
ÂÂÂ e.g,
ÂÂÂ fpsimd_save(), task_fpsimd_load() - save/restore task FP/SIMD
registers
ÂÂÂ fpsimd_bind_task_to_cpu()Â \
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ - Update the "state" metadata for
CPU/task.
ÂÂÂ fpsimd_bind_state_to_cpu() /
ÂÂÂ fpsimd_update_current_state() - Update the fp/simd state for the
current
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ task from memory.
ÂÂÂ These must not be called in the absence of FP/SIMD. Put in a
WARNING
ÂÂÂ to make sure they are not invoked in the absence of FP/SIMD.
KVM also uses the TIF_FOREIGN_FPSTATE flag to manage the FP/SIMD state
on the CPU. However, without FP/SIMD support we trap all accesses and
inject undefined instruction. Thus we should never "load" guest state.
Add a sanity check to make sure this is valid.
Yes, but no, see below.
Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD")
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
No idea who that guy is. It's a fake! ;-)
Sorry about that, will fix it.
Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
---
Âarch/arm64/kernel/fpsimd.c | 31 +++++++++++++++++++++++++++----
Âarch/arm64/kvm/hyp/switch.c |Â 9 +++++++++
Â2 files changed, 36 insertions(+), 4 deletions(-)
[...]
diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 72fbbd86eb5e..9696ebb5c13a 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -28,10 +28,19 @@
Â/* Check whether the FP regs were dirtied while in the host-side run
loop: */
Âstatic bool __hyp_text update_fp_enabled(struct kvm_vcpu *vcpu)
Â{
+ÂÂÂ /*
+ÂÂÂÂ * When the system doesn't support FP/SIMD, we cannot rely on
+ÂÂÂÂ * the state of _TIF_FOREIGN_FPSTATE. However, we will never
+ÂÂÂÂ * set the KVM_ARM64_FP_ENABLED, as the FP/SIMD accesses always
+ÂÂÂÂ * inject an abort into the guest. Thus we always trap the
+ÂÂÂÂ * accesses.
+ÂÂÂÂ */
ÂÂÂÂ if (vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE)
ÂÂÂÂÂÂÂÂ vcpu->arch.flags &= ~(KVM_ARM64_FP_ENABLED |
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ KVM_ARM64_FP_HOST);
+ÂÂÂ WARN_ON(!system_supports_fpsimd() &&
+ÂÂÂÂÂÂÂ (vcpu->arch.flags & KVM_ARM64_FP_ENABLED));
Careful, this will panic the host if it happens on a !VHE host
(calling non-inline stuff from a __hyp_text function is usually
not a good idea).
Ouch! Sorry about that WARN_ON()! I could drop the warning and
make this :
if (!system_supports_fpsimd() ||
ÂÂÂ (vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE))
ÂÂÂÂvcpu->arch.flags &= ~(KVM_ARM64_FP_ENABLED |
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ KVM_ARM64_FP_HOST);
to make sure we never say fp is enabled.
What do you think ?
Sure, that would work. I can't really see how KVM_ARM64_FP_ENABLED
Thanks I have fixed this locally now.
would get set though. But it probably doesn't matter (WTF is going
Right. That cannot be set to begin with, as the first access to FP/SIMD
injects an abort back to the guest, which is why I added a WARN() to
begin with.
Just wanted to be extra safe.
to run KVM with such broken HW?), and better safe than sorry.
Right, with no COMPAT KVM support it is really hard to get this far.
Cheers
Suzuki