linux-kernel - RE: [PATCH 1/6] x86/tdx: Support hypercalls for TDX guests on Hyper-V

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Message-ID: <SA1PR21MB13353C24B5BF2E7D6E8BCFA5BF0C9@SA1PR21MB1335.namprd21.prod.outlook.com>
Date:   Wed, 23 Nov 2022 01:37:26 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "arnd@...db.de" <arnd@...db.de>, "bp@...en8.de" <bp@...en8.de>,
        "brijesh.singh@....com" <brijesh.singh@....com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "jane.chu@...cle.com" <jane.chu@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/6] x86/tdx: Support hypercalls for TDX guests on Hyper-V

> From: Dave Hansen <dave.hansen@...el.com>
> Sent: Monday, November 21, 2022 12:39 PM
> [...]
> On 11/21/22 11:51, Dexuan Cui wrote:
> > __tdx_hypercall() doesn't work for a TDX guest running on Hyper-V,
> > because Hyper-V uses a different calling convention, so add the
> > new function __tdx_ms_hv_hypercall().
> 
> Other than R10 being variable here and fixed for __tdx_hypercall(), this
> looks *EXACTLY* the same as __tdx_hypercall(), or at least a strict
> subset of what __tdx_hypercall() can do.
> 
> Did I miss something?

The existing asm code for __tdx_hypercall() passes through R10~R15
(see TDVMCALL_EXPOSE_REGS_MASK) to the (KVM) hypervisor.

Unluckily, for Hyper-V, we need to pass through RDX, R8, R10 and R11
to Hyper-V, so I don't think I can use the existing __tdx_hypercall() ?

> Another way of saying this:  It seems like you could do this with a new
> version of _tdx_hypercall() (and all in C) instead of a new
> __tdx_hypercall().

I don't think the current TDVMCALL_EXPOSE_REGS_MASK allows me
to pass through RDX and R8 to Hyper-V.

PS, the comment before __tdx_hypercall() contains this line:

"* RBX, RBP, RDI, RSI  - Used to pass VMCALL sub function specific
arguments."

But it looks like currently RBX an RBP are not used at all in 
arch/x86/coco/tdx/tdcall.S ?