linux-kernel - Re: [PATCH] riscv: Add initial Xen guest support for RISC-V

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Message-Id: <D73F98QBJ4S9.3CN2ZUQ0GSMT6@ventanamicro.com>
Date: Thu, 16 Jan 2025 11:22:59 +0100
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Andrew Jones" <ajones@...tanamicro.com>,
 Milan Đokić <milandjokic1995@...il.com>
Cc: <linux-riscv@...ts.infradead.org>, <jgross@...e.com>,
 <aou@...s.berkeley.edu>, <Milan.Djokic@...rk.com>,
 <rafael.j.wysocki@...el.com>, <linux-kernel@...r.kernel.org>,
 <oleksandr_tyshchenko@...m.com>, <iommu@...ts.linux.dev>,
 <sstabellini@...nel.org>, <palmer@...belt.com>, <paul.walmsley@...ive.com>,
 <xen-devel@...ts.xenproject.org>, <Slavisa.Petrovic@...rk.com>,
 <takakura@...inux.co.jp>, "linux-riscv"
 <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH] riscv: Add initial Xen guest support for RISC-V

2025-01-16T09:51:25+01:00, Andrew Jones <ajones@...tanamicro.com>:
> On Wed, Jan 15, 2025 at 08:04:05PM +0100, Milan Đokić wrote:
>> On Tue, Jan 14, 2025 at 7:18 PM Andrew Jones <ajones@...tanamicro.com> wrote:
>> > On Tue, Jan 14, 2025 at 05:09:36PM +0100, Milan Djokic wrote:
>> > > +#define SBI_ECALL 0xE
>> >
>> > Shouldn't this be 0xA000007, i.e. the SBI firmware specific extension
>> > for Xen. Otherwise why refer to SBI? Note, '0xE' is an invalid, legacy
>> > extension ID in SBI.
>> >
>> Hypercall is triggered through SBI and we defined 0xE just as an
>> SBI_ECALL ID on Xen side for hypercall handling (among other operation
>> IDs), so we're not referring to some standard /legacy ID here, just
>> utilizing SBI for hypercall handling.
>
> If the SBI specified EIDs and binary encoding aren't used, then the
> hypercalls aren't "triggered through SBI", Xen is just doing its own
> thing on an ecall. Xen doesn't have to implement SBI at all, but if
> it wants to provide SBI services, as well as its own hypercalls, then
> the hypercalls should be encoded in the same way as SBI functions and
> an EID allowed by the SBI specification for hypervisor-specific
> functions should be used. For Xen, that EID is already specified and
> it's 0xA000007.

SBI specifies a complete calling convention, but it's not necessary for
binary compatibility.  SBI also aims to simplify caller API.

Linux maintainers will want a good reason for introducing separate Xen
SBI call functions/macros (linux already has sbi_ecall, so please try to
use and potentially improve it), but the ECALL is guaranteed to be SBI
compatible as long as a7=0xA000007.

a7 is needed to denote Xen's extension space and the remaining
input/output registers can be implemented in any way Xen wants.