linux-kernel - Re: [PATCH 4/6] virt: bao: Add Bao I/O dispatcher driver

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Message-ID: <4rrof4lk2zxut63u7qwxlmygpslvq5owfraatbjq3sbybtac4u@2prxnxijartx>
Date: Wed, 14 Jan 2026 14:32:38 -0600
From: Andrew Jones <andrew.jones@....qualcomm.com>
To: joaopeixoto@...x.tech
Cc: linux-kernel@...r.kernel.org, ajd@...ux.ibm.com, alex@...ti.fr,
        aou@...s.berkeley.edu, bagasdotme@...il.com, catalin.marinas@....com,
        conor+dt@...nel.org, corbet@....net, dan.j.williams@...el.com,
        davidmcerdeira@...x.tech, devicetree@...r.kernel.org, dev@...l-k.io,
        gregkh@...uxfoundation.org, haren@...ux.ibm.com, heiko@...ech.de,
        jose@...x.tech, kever.yang@...k-chips.com, krzk+dt@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-riscv@...ts.infradead.org, maddy@...ux.ibm.com, mani@...nel.org,
        nathan@...nel.org, neil.armstrong@...aro.org, palmer@...belt.com,
        pjw@...nel.org, prabhakar.mahadev-lad.rj@...renesas.com,
        robh@...nel.org, will@...nel.org
Subject: Re: [PATCH 4/6] virt: bao: Add Bao I/O dispatcher driver

On Wed, Jan 07, 2026 at 04:28:27PM +0000, joaopeixoto@...x.tech wrote:
...
> diff --git a/arch/riscv/include/asm/bao.h b/arch/riscv/include/asm/bao.h
> index 35658f37e1bd..f04e6cd33fa9 100644
> --- a/arch/riscv/include/asm/bao.h
> +++ b/arch/riscv/include/asm/bao.h
> @@ -14,6 +14,7 @@
>  #define __ASM_RISCV_BAO_H
>  
>  #include <asm/sbi.h>
> +#include <linux/bao.h>
>  
>  #define BAO_SBI_EXT_ID 0x08000ba0
>  
> @@ -28,4 +29,33 @@ static inline unsigned long bao_ipcshmem_hypercall(unsigned long hypercall_id,
>  	return ret.error;
>  }
>  
> +static inline unsigned long
> +bao_remio_hypercall(struct bao_remio_hypercall_ctx *ctx)
> +{
> +	register uintptr_t a0 asm("a0") = (uintptr_t)(ctx->dm_id);
> +	register uintptr_t a1 asm("a1") = (uintptr_t)(ctx->addr);
> +	register uintptr_t a2 asm("a2") = (uintptr_t)(ctx->op);
> +	register uintptr_t a3 asm("a3") = (uintptr_t)(ctx->value);
> +	register uintptr_t a4 asm("a4") = (uintptr_t)(ctx->request_id);
> +	register uintptr_t a5 asm("a5") = (uintptr_t)(0);
> +	register uintptr_t a6 asm("a6") = (uintptr_t)(BAO_REMIO_HYPERCALL_ID);
> +	register uintptr_t a7 asm("a7") = (uintptr_t)(0x08000ba0);
                                                      ^ BAO_SBI_EXT_ID

Using the experimental extension ID space would be fine for an RFC, but
this can't be merged until an SBI implementation ID for Bao is added to
the RISC-V SBI spec. Then the Bao EID would be '0xA000000 | <Bao-IMP-ID>'

I think we'll also need to discuss whether or not firmware/hypervisor-
specific extensions are exempt from all rules in chapter 3 of the SBI
spec other than a7 being the EID. If not, then this function should
just call __sbi_ecall() and the Bao hypercalls will not be allowed to
modify any registers except a0 and a1.

> +
> +	asm volatile("ecall"
> +		     : "+r"(a0), "+r"(a1), "+r"(a2), "+r"(a3), "+r"(a4),
> +		       "+r"(a5), "+r"(a6), "+r"(a7)
> +		     : "r"(a0), "r"(a1), "r"(a2), "r"(a3), "r"(a4), "r"(a5),
> +		       "r"(a6), "r"(a7)
> +		     : "memory");
> +
> +	ctx->addr = a2;
> +	ctx->op = a3;
> +	ctx->value = a4;
> +	ctx->access_width = a5;
> +	ctx->request_id = a6;
> +	ctx->npend_req = a7;
> +
> +	return a0;
> +}

Thanks,
drew