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Date: Thu, 01 Apr 2021 19:53:23 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v4 13/46] KVM: PPC: Book3S 64: Move GUEST_MODE_SKIP test
 into KVM
To: Paul Mackerras <paulus@ozlabs.org>
References: <20210323010305.1045293-1-npiggin@gmail.com>
 <20210323010305.1045293-14-npiggin@gmail.com>
 <YGVbApPydgwAU8cP@thinks.paulus.ozlabs.org>
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Cc: Fabiano Rosas <farosas@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org,
 kvm-ppc@vger.kernel.org, Daniel Axtens <dja@axtens.net>
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Excerpts from Paul Mackerras's message of April 1, 2021 3:32 pm:
> On Tue, Mar 23, 2021 at 11:02:32AM +1000, Nicholas Piggin wrote:
>> Move the GUEST_MODE_SKIP logic into KVM code. This is quite a KVM
>> internal detail that has no real need to be in common handlers.
>>=20
>> Also add a comment explaining why this thing exists.
>=20
> [snip]
>=20
>> diff --git a/arch/powerpc/kvm/book3s_64_entry.S b/arch/powerpc/kvm/book3=
s_64_entry.S
>> index 7a039ea78f15..a5412e24cc05 100644
>> --- a/arch/powerpc/kvm/book3s_64_entry.S
>> +++ b/arch/powerpc/kvm/book3s_64_entry.S
>> @@ -1,6 +1,7 @@
>>  /* SPDX-License-Identifier: GPL-2.0-only */
>>  #include <asm/asm-offsets.h>
>>  #include <asm/cache.h>
>> +#include <asm/exception-64s.h>
>>  #include <asm/kvm_asm.h>
>>  #include <asm/kvm_book3s_asm.h>
>>  #include <asm/ppc_asm.h>
>> @@ -20,9 +21,12 @@ kvmppc_interrupt:
>>  	 * guest R12 saved in shadow VCPU SCRATCH0
>>  	 * guest R13 saved in SPRN_SCRATCH0
>>  	 */
>> -#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
>>  	std	r9,HSTATE_SCRATCH2(r13)
>>  	lbz	r9,HSTATE_IN_GUEST(r13)
>> +	cmpwi	r9,KVM_GUEST_MODE_SKIP
>> +	beq-	.Lmaybe_skip
>> +.Lno_skip:
>> +#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
>>  	cmpwi	r9,KVM_GUEST_MODE_HOST_HV
>>  	beq	kvmppc_bad_host_intr
>>  #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
>> @@ -34,3 +38,48 @@ kvmppc_interrupt:
>>  #else
>>  	b	kvmppc_interrupt_pr
>>  #endif
>=20
> It's a bit hard to see without more context, but I think that in the
> PR-only case (CONFIG_KVM_BOOK3S_HV_POSSIBLE undefined), this will
> corrupt R9.  You need to restore R9 before the unconditional branch to
> kvmppc_interrupt_pr.  (I realize this code gets modified further, but
> I'd rather not break bisection.)

Very good catch, thanks.

Thanks,
Nick