linux-kernel - Re: [PATCH v2] iscsi_ibft: Fix finding the iBFT under Xen Dom 0

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Message-ID: <5aae0317-72cf-fc3c-6ce3-7e1fe1d6b699@intel.com>
Date:   Thu, 1 Jun 2023 09:57:30 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ross Lagerwall <ross.lagerwall@...rix.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:     Jan Beulich <jbeulich@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Peter Jones <pjones@...hat.com>,
        Konrad Rzeszutek Wilk <konrad@...nel.org>
Subject: Re: [PATCH v2] iscsi_ibft: Fix finding the iBFT under Xen Dom 0

On 5/30/23 08:01, Ross Lagerwall wrote:
> Since firmware doesn't indicate the iBFT in the E820, add a reserved
> region so that it gets identity mapped when running as Dom 0 so that it
> is possible to search for it. Move the call to reserve_ibft_region()
> later so that it is called after the Xen identity mapping adjustments
> are applied.
> 
> Finally, instead of using isa_bus_to_virt() which doesn't do the right
> thing under Xen, use early_memremap() like the dmi_scan code does.

This is connecting Xen, iSCSI and x86.  Some background here would be
*really* nice for dummies like me that deal heavily in only one of those
three.

One or two sentences like this:

	Firmware can provide an iSCSI-specific table called the iBFT
	which helps the OS boot from iSCSI devices.

can go a long way for dummies like me.  As could some background about
why this:

	... add a reserved region so that it gets identity mapped when
	running as Dom 0 so that it is possible to search for it.

These are all English words, but off the top of my head, I have no idea
why reserved regions get identity mapped when running as Dom 0 or why
that makes it possible to search.

The addresses and size here:

> +#ifdef CONFIG_ISCSI_IBFT_FIND
> +		/* Reserve 0.5 MiB to 1 MiB region so iBFT can be found */
> +		xen_e820_table.entries[xen_e820_table.nr_entries].addr = 0x80000;
> +		xen_e820_table.entries[xen_e820_table.nr_entries].size = 0x80000;
> +		xen_e820_table.entries[xen_e820_table.nr_entries].type = E820_TYPE_RESERVED;
> +		xen_e820_table.nr_entries++;
> +#endif

also appear to be conjured out of thin air.

As does the move of:

> +	reserve_ibft_region();

I'm sure I can go figure this all out with some research.  But, I'd
really appreciate some extra effort from you in this changelog to save
me the time.  I bet you can explain it a lot more efficiently than I can
go figure it out.