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Mon, 21 Jul 2025 17:20:03 -0700 (PDT) Message-ID: Date: Mon, 21 Jul 2025 17:20:01 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 2/5] KVM: arm64: Use SMCCC 1.2 for FF-A initialization and in host handler To: =?UTF-8?Q?Arve_Hj=C3=B8nnev=C3=A5g?= Cc: Will Deacon , Marc Zyngier , perlarsen@google.com, Oliver Upton , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Sudeep Holla , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, ahomescu@google.com, armellel@google.com, ayrton@google.com, qperret@google.com, sebastianene@google.com, qwandor@google.com References: <20250701-virtio-msg-ffa-v7-0-995afc3d385e@google.com> <20250701-virtio-msg-ffa-v7-2-995afc3d385e@google.com> <8634bdbgaz.wl-maz@kernel.org> <3b10fa81-bfdd-4325-a330-c79df2e21621@immunant.com> <67cc6766-6493-4171-a6b1-2a105c3e6ff5@immunant.com> Content-Language: en-US From: Per Larsen In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 7/21/25 4:01 AM, Arve Hjønnevåg wrote: > On Fri, Jul 18, 2025 at 10:54 PM Per Larsen wrote: >> >> On 7/18/25 6:37 AM, Will Deacon wrote: >>> On Mon, Jul 07, 2025 at 05:06:23PM -0700, Per Larsen wrote: >>>> On 7/3/25 5:38 AM, Marc Zyngier wrote: >>>>> On Tue, 01 Jul 2025 23:06:35 +0100, >>>>> Per Larsen via B4 Relay wrote: >>>>>> diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c >>>>>> index 2c199d40811efb5bfae199c4a67d8ae3d9307357..65d241ba32403d014b43cc4ef4d5bf9693813809 100644 >>>>>> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c >>>>>> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c >>>>>> @@ -71,36 +71,68 @@ static u32 hyp_ffa_version; >>>>>> static bool has_version_negotiated; >>>>>> static hyp_spinlock_t version_lock; >>>>>> -static void ffa_to_smccc_error(struct arm_smccc_res *res, u64 ffa_errno) >>>>>> +static void ffa_to_smccc_error(struct arm_smccc_1_2_regs *res, u64 ffa_errno) >>>>>> { >>>>>> - *res = (struct arm_smccc_res) { >>>>>> + *res = (struct arm_smccc_1_2_regs) { >>>>>> .a0 = FFA_ERROR, >>>>>> .a2 = ffa_errno, >>>>>> }; >>>>>> } >>>>>> -static void ffa_to_smccc_res_prop(struct arm_smccc_res *res, int ret, u64 prop) >>>>>> +static void ffa_to_smccc_res_prop(struct arm_smccc_1_2_regs *res, int ret, u64 prop) >>>>>> { >>>>>> if (ret == FFA_RET_SUCCESS) { >>>>>> - *res = (struct arm_smccc_res) { .a0 = FFA_SUCCESS, >>>>>> - .a2 = prop }; >>>>>> + *res = (struct arm_smccc_1_2_regs) { .a0 = FFA_SUCCESS, >>>>>> + .a2 = prop }; >>>>>> } else { >>>>>> ffa_to_smccc_error(res, ret); >>>>>> } >>>>>> } >>>>>> -static void ffa_to_smccc_res(struct arm_smccc_res *res, int ret) >>>>>> +static void ffa_to_smccc_res(struct arm_smccc_1_2_regs *res, int ret) >>>>>> { >>>>>> ffa_to_smccc_res_prop(res, ret, 0); >>>>>> } >>>>>> static void ffa_set_retval(struct kvm_cpu_context *ctxt, >>>>>> - struct arm_smccc_res *res) >>>>>> + struct arm_smccc_1_2_regs *res) >>>>>> { >>>>>> + DECLARE_REG(u64, func_id, ctxt, 0); >>>>>> cpu_reg(ctxt, 0) = res->a0; >>>>>> cpu_reg(ctxt, 1) = res->a1; >>>>>> cpu_reg(ctxt, 2) = res->a2; >>>>>> cpu_reg(ctxt, 3) = res->a3; >>>>>> + cpu_reg(ctxt, 4) = res->a4; >>>>>> + cpu_reg(ctxt, 5) = res->a5; >>>>>> + cpu_reg(ctxt, 6) = res->a6; >>>>>> + cpu_reg(ctxt, 7) = res->a7; >>>>> >>>>> From DEN0028G 2.6: >>>>> >>>>> >>>>> Registers W4-W7 must be preserved unless they contain results, as >>>>> specified in the function definition. >>>>> >>>>> >>>>> On what grounds can you blindly change these registers? >>>> From DEN0077A 1.2 Section 11.2: Reserved parameter convention >>>> >>>> >>>> Unused parameter registers in FF-A ABIs are reserved for future use by the >>>> Framework. >>>> >>>> [...] >>>> >>>> The caller is expected to write zeroes to these registers. The callee >>>> ignores the values in these registers. >>>> >>>> >>>> My read is that, as long as we are writing zeroes into reserved registers >>>> (which I believe we are), we comply with DEN0026G 2.6.> >>> >>> Right, the specs make this far more difficult to decipher than necessary >>> but I think SMCCC defers to the definition of the specific call being >>> made and then FF-A is basically saying that unused argument registers >>> are always zeroed. >>> >>> Rather than have the EL2 proxy treat each call differently based on the >>> used argument registers, we can rely on EL3 doing the right thing and >>> blindly copy everything back, which is what you've done. So I think >>> that's ok. >>> >>>>>> + >>>>>> + /* >>>>>> + * DEN0028C 2.6: SMC32/HVC32 call from aarch64 must preserve x8-x30. >>>>>> + * >>>>>> + * The most straightforward approach is to look at the function ID >>>>>> + * sent by the caller. However, the caller could send FFA_MSG_WAIT >>>>>> + * which is a 32-bit interface but the reply could very well be 64-bit >>>>>> + * such as FFA_FN64_MSG_SEND_DIRECT_REQ or FFA_MSG_SEND_DIRECT_REQ2. >>>>>> + * >>>>>> + * Instead, we could look at the function ID in the response (a0) but >>>>>> + * that doesn't work either as FFA_VERSION responses put the version >>>>>> + * number (or error code) in w0. >>>>>> + * >>>>>> + * Set x8-x17 iff response contains 64-bit function ID in a0. >>>>>> + */ >>>>>> + if (func_id != FFA_VERSION && ARM_SMCCC_IS_64(res->a0)) { >>>>>> + cpu_reg(ctxt, 8) = res->a8; >>>>>> + cpu_reg(ctxt, 9) = res->a9; >>>>>> + cpu_reg(ctxt, 10) = res->a10; >>>>>> + cpu_reg(ctxt, 11) = res->a11; >>>>>> + cpu_reg(ctxt, 12) = res->a12; >>>>>> + cpu_reg(ctxt, 13) = res->a13; >>>>>> + cpu_reg(ctxt, 14) = res->a14; >>>>>> + cpu_reg(ctxt, 15) = res->a15; >>>>>> + cpu_reg(ctxt, 16) = res->a16; >>>>>> + cpu_reg(ctxt, 17) = res->a17; >>>>>> + } >>>>>> } >>>>> >>>>> I don't see how that can ever work. >>>>> >>>>> Irrespective of how FFA_MSG_WAIT actually works (and I couldn't find >>>>> anything in the spec that supports the above), the requester will >>>>> fully expect its registers to be preserved based on the initial >>>>> function type, and that alone. No ifs, no buts. >>>>> >>>>> If what you describe can happen (please provide a convincing example), >>>>> then the spec is doomed. >>>> >>>> DEN0077A 1.2 Section 8.2 (Runtime Model for FFA_RUN) and 8.3 (Runtime Model >>>> for Direct request ABIs) contains Figures 8.1 and 8.2. Each figure shows >>>> transitions between states "waiting", "blocked", "running", and "preempted". >>>> Key to my understanding is that the waiting state in Figure 8.1 and Figure >>>> 8.2 is the exact same state. This appears to be the case per Section 4.10. >>>> >>>> So we have to consider the ways to get in and out of the waiting state by >>>> joining the state machines in Figures 8.1 and 8.2. Figure 8.1 has an edge >>>> between "running" and "waiting" caused by FFA_MSG_WAIT. Figure 8.2 has an >>>> edge between "waiting" and "running" caused by a "Direct request ABI". >>>> >>>> Direct request ABIs include FFA_MSG_SEND_DIRECT_REQ2 which is why the FF-A >>>> 1.2 spec, in my read, permits the response to a 32-bit FFA_MSG_WAIT call can >>>> be a 64-bit FFA_MSG_SEND_DIRECT_REQ2 reply. >>> >>> That seems bonkers to me and I agree with Marc's assessment above. The >>> SMCCC is crystal clear that a caller using the SM32/HVC32 calling >>> convention can rely on x8-x30 (as well as the stack pointers) being >>> preserved. If FF-A has a problem with that then it needs to be fixed >>> there and not bodged in Linux. >> Ack. Patchset v8 addresses Marc's feedback by using the func_id detect >> SMC64 instead.> >>> Setting that aside, I'm still not sure I follow this part of your check: >>> >>> if (... && ARM_SMCCC_IS_64(res->a0)) >>> >>> won't res->a0 contain something like FFA_SUCCESS? The FFA spec states: >>> >>> FFA_SUCCESS64, is used only if any result register encodes a 64-bit >>> parameter. >>> >>> which doesn't really seem related to whether or not the initial call >>> was using SMC32 or SMC64. What am I missing?I agree that we cannot use res->a0 to distinguish SMC32/SMC64 for the >> reason you stated. > > I don't think using the function-id of the original call works > correctly in this context though. If you look at > drivers/firmware/arm_ffa/driver.c:ffa_msg_send_direct_req2 it has the > same problem as the FFA_MSG_WAIT example in your comment. In the > simple case it will use FFA_MSG_SEND_DIRECT_REQ2 for the call and > FFA_MSG_SEND_DIRECT_RESP2 for the response, both 64 bit opcodes, and > either approach here will have the same correct result. However if > FFA_MSG_SEND_DIRECT_REQ2 responds with FFA_INTERRUPT or FFA_YIELD, > then the driver will resume the call with FFA_RUN, a 32 bit opcode, > and still expect a 64 bit FFA_MSG_SEND_DIRECT_RESP2 response with a > full response in x4-17. If you use ARM_SMCCC_IS_64(func_id) here > instead of ARM_SMCCC_IS_64(res->a0), then the part of response in > x8-x17 will be lost. > > The FF-A 1.3 ALP2 fixes this by adding a 64 bit FF-A run opcode, but > at the current patchstack only adds ff-a 1.2 support and the linux > ff-a driver does not yet support the new 1.3 ALP2 call flow either so > I think the current v7 patch here is the best option for now. > FFA_RUN is passed through to EL3 by kvm_host_ffa_handler so I'm not sure there is a code path where func_id == FFA_RUN in set_ffa_retval.