linux-kernel - Re: [PATCH v15 2/3] virt: Add TDX guest driver

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Message-ID: <f03f1db3-5e55-9606-1d0d-4d51213a0b1a@intel.com>
Date:   Mon, 24 Oct 2022 07:17:22 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Shuah Khan <shuah@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Kai Huang <kai.huang@...el.com>,
        Wander Lairson Costa <wander@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v15 2/3] virt: Add TDX guest driver

On 10/23/22 09:13, Sathyanarayanan Kuppuswamy wrote:
> On 10/20/22 9:39 PM, Greg Kroah-Hartman wrote:
>>>> You are allowing userspace to spam the kernel logs, please do not do
>>>> that.
>>> Added it to help userspace understand the reason for the failure (only for
>>> the cases like request param issues and TDCALL failure). Boris recommended
>>> adding it in the previous review.
>> Again, you just created a vector for userspace to spam the kernel log.
>> No kernel driver should ever do that.
>>
> Brois, any comments? Do you also agree?
...
> +	if (req.subtype || req.rpd_len != TDX_REPORTDATA_LEN ||
> +	    req.tdr_len != TDX_REPORT_LEN) {
> +		pr_err("TDX_CMD_GET_REPORT: invalid req: subtype:%u rpd_len:%u tdr_len:%u\n",
> +		       req.subtype, req.rpd_len, req.tdr_len);

This is _clearly_ debugging code.  There are a billion if(foo){return
-EINVAL;}'s in the kernel, and very few of them have printk()'s to go
along with them.

They do help figure out what happened when userspace sees an -EINVAL and
can't figure out what it did to cause it.  But, if the kernel spammed
dmesg for every time userspace does something stupid, it'd be filled up
with noise.

There are other ways to debug stuff like this if userspace gets confused.

If folks are OK with dev_dbg(), then I'd move over to that.  But,
frankly, I don't think this rises to the level of needing its own error
message.

Heck, I'm not even sure why this code exits in the first place.  I guess
we don't want userspace making random requests to the host.  But, of
course, none of _that_ information about what the code is actually there
for made it into the patch, and it just consumes comment space
regurgitating the TDX spec.

This branch of the thread frankly isn't about a pr_err().  It's about
nobody really knowing (or caring) why that line of code is there, when
it might happen, and what precise function it serves.