linux-kernel - Re: [PATCH 1/2] x86/tdx: Extract GET_INFO call from get_cc

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Date:   Mon, 31 Oct 2022 09:42:15 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, ak@...ux.intel.com,
        dan.j.williams@...el.com, david@...hat.com, hpa@...or.com,
        seanjc@...gle.com, thomas.lendacky@....com,
        elena.reshetova@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/tdx: Extract GET_INFO call from get_cc_mask()

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?