[PATCH] arm64: KVM: VHE: save and restore some PSTATE bits

[gengdongjiu]

Hi Marc,

On 2017/9/6 16:17, Marc Zyngier wrote:
> On 05/09/17 19:58, gengdongjiu wrote:
>> when exit from guest, some host PSTATE bits may be lost, such as
>> PSTATE.PAN or PSTATE.UAO. It is because host and hypervisor all run
>> in the EL2, host PSTATE value cannot be saved and restored via
>> SPSR_EL2. So if guest has changed the PSTATE, host continues with
>> a wrong value guest has set.
>>
>> Signed-off-by: Dongjiu Geng <gengdongjiu at huawei.com>
>> Signed-off-by: Haibin Zhang <zhanghaibin7 at huawei.com>
>> ---
>>  arch/arm64/include/asm/kvm_host.h |  8 +++++++
>>  arch/arm64/include/asm/kvm_hyp.h  |  2 ++
>>  arch/arm64/include/asm/sysreg.h   | 23 +++++++++++++++++++
>>  arch/arm64/kvm/hyp/entry.S        |  2 --
>>  arch/arm64/kvm/hyp/switch.c       | 24 ++++++++++++++++++--
>>  arch/arm64/kvm/hyp/sysreg-sr.c    | 48 ++++++++++++++++++++++++++++++++++++---
>>  6 files changed, 100 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index e923b58..cba7d3e 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -193,6 +193,12 @@ struct kvm_cpu_context {
>>  	};
>>  };
>>  
>> +struct kvm_cpu_host_pstate {
>> +	u64 daif;
>> +	u64 uao;
>> +	u64 pan;
>> +};
> 
> I love it. This is the most expensive way of saving/restoring a single
> 32bit value.
> 
> More seriously, please see the discussion between James and Christoffer
> there[1]. I expect James to address the PAN/UAO states together with the
> debug state in the next iteration of his patch.

I roughly see the discussion between James and Christoffer, Seems Christoffer does not suggest save and
restore it, and suggest to do below, and UAO/PAN may not use the same ways.

      __kvm_vcpu_run(struct kvm_vcpu *vcpu)
      {
          if (has_vhe())
              asm("msr     daifset, #0xf");

 	...
         exit_code = __guest_enter(vcpu, host_ctxt);
 	...

 	if (has_vhe())
              asm("msr     daifclr, #0xd");
      }


If not save/restore them, the KVM will set them according to the CPU capability. For example below fixing, it will check CPU capability, If CPU supports PAN,
the kvm will always enable the PAN for the host. But in fact, the host may be not enable the PAN.
Of course for the UAO, we can use the similar fixing if Marc or Christoffer is agreed. but seems not make sense.

commit cb96408da4e11698674abd04aeac941c1bed2038
Author: Vladimir Murzin <vladimir.murzin at arm.com>
Date:   Thu Sep 1 15:29:03 2016 +0100

    arm64: KVM: VHE: reset PSTATE.PAN on entry to EL2

    SCTLR_EL2.SPAN bit controls what happens with the PSTATE.PAN bit on an
    exception. However, this bit has no effect on the PSTATE.PAN when
    HCR_EL2.E2H or HCR_EL2.TGE is unset. Thus when VHE is used and
    exception taken from a guest PSTATE.PAN bit left unchanged and we
    continue with a value guest has set.

    To address that always reset PSTATE.PAN on entry from EL1.

    Fixes: 1f364c8c48a0 ("arm64: VHE: Add support for running Linux in EL2 mode")

    Signed-off-by: Vladimir Murzin <vladimir.murzin at arm.com>
    Reviewed-by: James Morse <james.morse at arm.com>
    Acked-by: Marc Zyngier <marc.zyngier at arm.com>
    Cc: <stable at vger.kernel.org> # v4.6+
    Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index 3967c231..b5926ee 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -96,6 +96,8 @@ ENTRY(__guest_exit)

        add     x1, x1, #VCPU_CONTEXT

+       ALTERNATIVE(nop, SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN)
+
        // Store the guest regs x2 and x3
        stp     x2, x3,   [x1, #CPU_XREG_OFFSET(2)]


> 
> Thanks,
> 
> 	M.
> 
> [1] https://www.spinics.net/lists/arm-kernel/msg599798.html
>