Re: [PATCH 1/2] x86/tdx: Extract GET_INFO call from get_cc_mask()

From: Dave Hansen <dave.hansen@intel.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
	luto@kernel.org, peterz@infradead.org,
	sathyanarayanan.kuppuswamy@linux.intel.com, ak@linux.intel.com,
	dan.j.williams@intel.com, david@redhat.com, hpa@zytor.com,
	seanjc@google.com, thomas.lendacky@amd.com,
	elena.reshetova@intel.com, x86@kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] x86/tdx: Extract GET_INFO call from get_cc_mask()
Date: Mon, 31 Oct 2022 09:42:15 -0700	[thread overview]
Message-ID: <812cb868-1729-8d78-cf91-1e63c7933fae@intel.com> (raw)
In-Reply-To: <20221031041252.mdcjocqn6k4k4gvy@box.shutemov.name>

On 10/30/22 21:12, Kirill A. Shutemov wrote:
> On Sat, Oct 29, 2022 at 02:59:51AM +0300, Kirill A. Shutemov wrote:
>>> Can you please take a look through this and make sure I didn't botch
>>> anything:
>>>
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/daveh/devel.git/log/?h=tdxbadve
>>>
>>> The end result is about 50 lines less than what was there before.  Most
>>> of it is comment removal but the code is simpler too.
>>>
>>> Acks and Tested-by's would be appreciated.
> 
> One thing that I must bring up is that it seems that there's no way to get
> the panic message to user. I tried to convinced myself that it is qemu
> misconfiguration on my part or some race, but no: it is just too early for
> earlyprintk.
> 
> We only get earlyprintk working after parse_early_options() which happens
> well after tdx_early_init().
> 
> Moving panic() after earlyprintk working is not good idea as it exposes
> kernel more: by the time we already have full #VE handler.

How about we soften the panic() to a pr_err() if it's a debug guest?

The first thing a user is going to do if they get an early boot failure
is flip the debug switch and try it again.  That gets us safe,
well-defined behavior when we need security and also lets us figure out
what went wrong.

Also, did anyone ever actually implement that TDX earlyprintk simple
console thing?  A TDCALL up to the host with some characters in a
register or two is as dirt simple of a console as you can get.  It would
be very easy to improve the user experience here if there were a:

	tdx_puts("uh oh");

interface.  It's a shame if it didn't get done by now.  I asked for it
years ago.

And, yeah, I know it wouldn't help us in this precise situation because
earlyprintk doesn't work yet.  But, it *would* be one of those really,
really early bitbanging-style consoles that _could_ be in use very, very
early if the printk() infrastructure could take advantage of it.

> We can move it earlier into decompresser which has different earlyprintk
> implementation. Not sure if it worth this. What do you think?

There's the puts()/printf() gunk that's really early like in
validate_cpu().  Is that what you were thinking of?