linux-kernel - Re: [PATCH v3 4/5] x86/tdx: tdx_mcall_get_report0: Return -EBUSY on TDCALL_OPERAND

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Message-ID: <7d1330e3-a0f0-4a4c-a487-46304822e0fd@intel.com>
Date: Fri, 11 Apr 2025 12:37:33 -0500
From: "Xing, Cedric" <cedric.xing@...el.com>
To: Dan Williams <dan.j.williams@...el.com>, "Kirill A. Shutemov"
	<kirill.shutemov@...ux.intel.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, <x86@...nel.org>, "H. Peter Anvin"
	<hpa@...or.com>
CC: <linux-kernel@...r.kernel.org>, <linux-coco@...ts.linux.dev>, "Dionna
 Amalie Glaze" <dionnaglaze@...gle.com>, Guorui Yu
	<guorui.yu@...ux.alibaba.com>, James Bottomley
	<James.Bottomley@...senpartnership.com>, Dan Middleton
	<dan.middleton@...ux.intel.com>, Mikko Ylinen <mikko.ylinen@...ux.intel.com>,
	Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH v3 4/5] x86/tdx: tdx_mcall_get_report0: Return -EBUSY on
 TDCALL_OPERAND_BUSY error

On 4/9/2025 12:13 AM, Dan Williams wrote:
> Cedric Xing wrote:
>> Return `-EBUSY` from tdx_mcall_get_report0() when `TDG.MR.REPORT` returns
>> `TDCALL_OPERAND_BUSY`. This enables the caller to retry obtaining a
>> TDREPORT later if another VCPU is extending an RTMR concurrently.
> 
> Can this not be prevented by proper locking? Otherwise this type of
> collision sounds like a kernel bug, not something that should escape to
> userspace.
> 
Yes, -EBUSY should never happen with proper locking, which however is 
implemented in the upper layer.

Similarly, -EINVAL will also indicate a kernel bug but is left for the 
upper layer to handle.

> I.e. userspace can not reasonably know when it is safe to retry, so take
> locks to ensure forward progress.
tdx-guest does WARN() on errors. There are no other users of this 
function currently. Returning an error, however, will allow different 
error handling in the future (e.g., retry instead of WARN on -EBUSY).