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Message-Id: <D1Q54PY40E3B.22QS5DMQRA58N@gmail.com>
Date: Mon, 03 Jun 2024 15:42:22 +1000
From: "Nicholas Piggin" <npiggin@...il.com>
To: "Gautam Menghani" <gautam@...ux.ibm.com>, <mpe@...erman.id.au>,
 <christophe.leroy@...roup.eu>, <aneesh.kumar@...nel.org>,
 <naveen.n.rao@...ux.ibm.com>, <corbet@....net>
Cc: <linuxppc-dev@...ts.ozlabs.org>, <kvm@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
 <stable@...r.kernel.org>
Subject: Re: [PATCH v1 RESEND] arch/powerpc/kvm: Fix doorbell emulation by
 adding DPDES support

On Wed May 22, 2024 at 6:49 PM AEST, Gautam Menghani wrote:
> Doorbell emulation is broken for KVM on PowerVM guests as support for
> DPDES was not added in the initial patch series. Due to this, a KVM on
> PowerVM guest cannot be booted with the XICS interrupt controller as
> doorbells are to be setup in the initial probe path when using XICS
> (pSeries_smp_probe()). Add DPDES support in the host KVM code to fix
> doorbell emulation.

This is broken when the KVM guest has SMT > 1? Or is it broken for SMT=1
as well? Can you explain a bit more of what breaks if it's the latter?

> Fixes: 6ccbbc33f06a ("KVM: PPC: Add helper library for Guest State Buffers")
> Cc: stable@...r.kernel.org
> Signed-off-by: Gautam Menghani <gautam@...ux.ibm.com>
> ---
> v1 -> v1 resend:
> 1. Add the stable tag
>
>  Documentation/arch/powerpc/kvm-nested.rst     |  4 +++-
>  arch/powerpc/include/asm/guest-state-buffer.h |  3 ++-
>  arch/powerpc/include/asm/kvm_book3s.h         |  1 +
>  arch/powerpc/kvm/book3s_hv.c                  | 14 +++++++++++++-
>  arch/powerpc/kvm/book3s_hv_nestedv2.c         |  7 +++++++
>  arch/powerpc/kvm/test-guest-state-buffer.c    |  2 +-
>  6 files changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/arch/powerpc/kvm-nested.rst b/Documentation/arch/powerpc/kvm-nested.rst
> index 630602a8aa00..5defd13cc6c1 100644
> --- a/Documentation/arch/powerpc/kvm-nested.rst
> +++ b/Documentation/arch/powerpc/kvm-nested.rst
> @@ -546,7 +546,9 @@ table information.
>  +--------+-------+----+--------+----------------------------------+
>  | 0x1052 | 0x08  | RW |   T    | CTRL                             |
>  +--------+-------+----+--------+----------------------------------+
> -| 0x1053-|       |    |        | Reserved                         |
> +| 0x1053 | 0x08  | RW |   T    | DPDES                            |
> ++--------+-------+----+--------+----------------------------------+
> +| 0x1054-|       |    |        | Reserved                         |
>  | 0x1FFF |       |    |        |                                  |
>  +--------+-------+----+--------+----------------------------------+
>  | 0x2000 | 0x04  | RW |   T    | CR                               |
> diff --git a/arch/powerpc/include/asm/guest-state-buffer.h b/arch/powerpc/include/asm/guest-state-buffer.h
> index 808149f31576..d107abe1468f 100644
> --- a/arch/powerpc/include/asm/guest-state-buffer.h
> +++ b/arch/powerpc/include/asm/guest-state-buffer.h
> @@ -81,6 +81,7 @@
>  #define KVMPPC_GSID_HASHKEYR			0x1050
>  #define KVMPPC_GSID_HASHPKEYR			0x1051
>  #define KVMPPC_GSID_CTRL			0x1052
> +#define KVMPPC_GSID_DPDES			0x1053
>  
>  #define KVMPPC_GSID_CR				0x2000
>  #define KVMPPC_GSID_PIDR			0x2001
> @@ -110,7 +111,7 @@
>  #define KVMPPC_GSE_META_COUNT (KVMPPC_GSE_META_END - KVMPPC_GSE_META_START + 1)
>  
>  #define KVMPPC_GSE_DW_REGS_START KVMPPC_GSID_GPR(0)
> -#define KVMPPC_GSE_DW_REGS_END KVMPPC_GSID_CTRL
> +#define KVMPPC_GSE_DW_REGS_END KVMPPC_GSID_DPDES
>  #define KVMPPC_GSE_DW_REGS_COUNT \
>  	(KVMPPC_GSE_DW_REGS_END - KVMPPC_GSE_DW_REGS_START + 1)
>  
> diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
> index 3e1e2a698c9e..10618622d7ef 100644
> --- a/arch/powerpc/include/asm/kvm_book3s.h
> +++ b/arch/powerpc/include/asm/kvm_book3s.h
> @@ -594,6 +594,7 @@ static inline u##size kvmppc_get_##reg(struct kvm_vcpu *vcpu)		\
>  
>  
>  KVMPPC_BOOK3S_VCORE_ACCESSOR(vtb, 64, KVMPPC_GSID_VTB)
> +KVMPPC_BOOK3S_VCORE_ACCESSOR(dpdes, 64, KVMPPC_GSID_DPDES)
>  KVMPPC_BOOK3S_VCORE_ACCESSOR_GET(arch_compat, 32, KVMPPC_GSID_LOGICAL_PVR)
>  KVMPPC_BOOK3S_VCORE_ACCESSOR_GET(lpcr, 64, KVMPPC_GSID_LPCR)
>  KVMPPC_BOOK3S_VCORE_ACCESSOR_SET(tb_offset, 64, KVMPPC_GSID_TB_OFFSET)
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 35cb014a0c51..cf285e5153ba 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -4116,6 +4116,11 @@ static int kvmhv_vcpu_entry_nestedv2(struct kvm_vcpu *vcpu, u64 time_limit,
>  	int trap;
>  	long rc;
>  
> +	if (vcpu->arch.doorbell_request) {
> +		vcpu->arch.doorbell_request = 0;
> +		kvmppc_set_dpdes(vcpu, 1);
> +	}

This probably looks okay... hmm, is the v1 KVM emulating doorbells
correctly for SMT L2 guests? I wonder if doorbell emulation isn't
broken there too because the L1 code looks to be passing in vc->dpdes
but all the POWER9 emulation code uses doorbell_request.

> +
>  	io = &vcpu->arch.nestedv2_io;
>  
>  	msr = mfmsr();
> @@ -4278,9 +4283,16 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
>  	if (kvmhv_on_pseries()) {
>  		if (kvmhv_is_nestedv1())
>  			trap = kvmhv_vcpu_entry_p9_nested(vcpu, time_limit, lpcr, tb);
> -		else
> +		else {
>  			trap = kvmhv_vcpu_entry_nestedv2(vcpu, time_limit, lpcr, tb);
>  
> +			/* Remember doorbell if it is pending  */
> +			if (kvmppc_get_dpdes(vcpu)) {
> +				vcpu->arch.doorbell_request = 1;
> +				kvmppc_set_dpdes(vcpu, 0);
> +			}

This is adding an extra get state for every entry, not good. I don't
think it's actually needed though. I don't think the L1 cares at this
stage what the L2 DPDES state is. So you sholud be able to drop this
hunk.

> +		}
> +
>  		/* H_CEDE has to be handled now, not later */
>  		if (trap == BOOK3S_INTERRUPT_SYSCALL && !nested &&
>  		    kvmppc_get_gpr(vcpu, 3) == H_CEDE) {
> diff --git a/arch/powerpc/kvm/book3s_hv_nestedv2.c b/arch/powerpc/kvm/book3s_hv_nestedv2.c
> index 8e6f5355f08b..36863fff2a99 100644
> --- a/arch/powerpc/kvm/book3s_hv_nestedv2.c
> +++ b/arch/powerpc/kvm/book3s_hv_nestedv2.c
> @@ -311,6 +311,10 @@ static int gs_msg_ops_vcpu_fill_info(struct kvmppc_gs_buff *gsb,
>  			rc = kvmppc_gse_put_u64(gsb, iden,
>  						vcpu->arch.vcore->vtb);
>  			break;
> +		case KVMPPC_GSID_DPDES:
> +			rc = kvmppc_gse_put_u64(gsb, iden,
> +						vcpu->arch.vcore->dpdes);
> +			break;
>  		case KVMPPC_GSID_LPCR:
>  			rc = kvmppc_gse_put_u64(gsb, iden,
>  						vcpu->arch.vcore->lpcr);
> @@ -543,6 +547,9 @@ static int gs_msg_ops_vcpu_refresh_info(struct kvmppc_gs_msg *gsm,
>  		case KVMPPC_GSID_VTB:
>  			vcpu->arch.vcore->vtb = kvmppc_gse_get_u64(gse);
>  			break;
> +		case KVMPPC_GSID_DPDES:
> +			vcpu->arch.vcore->dpdes = kvmppc_gse_get_u64(gse);
> +			break;
>  		case KVMPPC_GSID_LPCR:
>  			vcpu->arch.vcore->lpcr = kvmppc_gse_get_u64(gse);
>  			break;

I would split all the wiring up of the DPDES GSID stuff into its own
patch, it obviously looks fine.

> diff --git a/arch/powerpc/kvm/test-guest-state-buffer.c b/arch/powerpc/kvm/test-guest-state-buffer.c
> index 4720b8dc8837..91ae660cfe21 100644
> --- a/arch/powerpc/kvm/test-guest-state-buffer.c
> +++ b/arch/powerpc/kvm/test-guest-state-buffer.c
> @@ -151,7 +151,7 @@ static void test_gs_bitmap(struct kunit *test)
>  		i++;
>  	}
>  
> -	for (u16 iden = KVMPPC_GSID_GPR(0); iden <= KVMPPC_GSID_CTRL; iden++) {
> +	for (u16 iden = KVMPPC_GSID_GPR(0); iden <= KVMPPC_GSID_DPDES; iden++) {
>  		kvmppc_gsbm_set(&gsbm, iden);
>  		kvmppc_gsbm_set(&gsbm1, iden);
>  		KUNIT_EXPECT_TRUE(test, kvmppc_gsbm_test(&gsbm, iden));

It would be good to have a  _LAST define for such loops. It's very easy
to miss when adding KVMPPC_GSID_DPDES that you need to grep for
KVMPPC_GSID_CTRL. Very easy to see that you need to update _LAST.

You just need to work out a good name for it since there's a few
"namespaces" of numbers with similar prefix. Good luck :)

Thanks,
Nick

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