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Date: Tue, 13 Apr 2021 11:25:51 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v1 01/12] KVM: PPC: Book3S HV P9: Restore host CTRL SPR
 after guest exit
To: Fabiano Rosas <farosas@linux.ibm.com>, kvm-ppc@vger.kernel.org
References: <20210412014845.1517916-1-npiggin@gmail.com>
 <20210412014845.1517916-2-npiggin@gmail.com> <877dl761iv.fsf@linux.ibm.com>
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Excerpts from Fabiano Rosas's message of April 13, 2021 12:06 am:
> Nicholas Piggin <npiggin@gmail.com> writes:
>=20
>> The host CTRL (runlatch) value is not restored after guest exit. The
>> host CTRL should always be 1 except in CPU idle code, so this can result
>> in the host running with runlatch clear, and potentially switching to
>> a different vCPU which then runs with runlatch clear as well.
>>
>> This has little effect on P9 machines, CTRL is only responsible for some
>> PMU counter logic in the host and so other than corner cases of software
>> relying on that, or explicitly reading the runlatch value (Linux does
>> not appear to be affected but it's possible non-Linux guests could be),
>> there should be no execution correctness problem, though it could be
>> used as a covert channel between guests.
>>
>> There may be microcontrollers, firmware or monitoring tools that sample
>> the runlatch value out-of-band, however since the register is writable
>> by guests, these values would (should) not be relied upon for correct
>> operation of the host, so suboptimal performance or incorrect reporting
>> should be the worst problem.
>>
>> Fixes: 95a6432ce9038 ("KVM: PPC: Book3S HV: Streamlined guest entry/exit=
 path on P9 for radix guests")
>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>>  arch/powerpc/kvm/book3s_hv.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
>> index 13bad6bf4c95..208a053c9adf 100644
>> --- a/arch/powerpc/kvm/book3s_hv.c
>> +++ b/arch/powerpc/kvm/book3s_hv.c
>> @@ -3728,7 +3728,10 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *=
vcpu, u64 time_limit,
>>  	vcpu->arch.dec_expires =3D dec + tb;
>>  	vcpu->cpu =3D -1;
>>  	vcpu->arch.thread_cpu =3D -1;
>> +	/* Save guest CTRL register, set runlatch to 1 */
>>  	vcpu->arch.ctrl =3D mfspr(SPRN_CTRLF);
>> +	if (!(vcpu->arch.ctrl & 1))
>> +		mtspr(SPRN_CTRLT, vcpu->arch.ctrl | 1);
>=20
> Maybe ditch the comment and use the already defined CTRL_RUNLATCH?

I did it this way so you can more easily match up the C with the=20
existing asm version.

I have a later patch to clean up CTRL handling a bit (in both C and=20
asm).

Thanks,
Nick