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

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Date:   Mon, 5 Jun 2023 10:27:49 +0000
From:   Ross Lagerwall <ross.lagerwall@...rix.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.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" <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

> From: Dave Hansen <dave.hansen@...el.com>
> Sent: Thursday, June 1, 2023 5:57 PM
> To: Ross Lagerwall <ross.lagerwall@...rix.com>; linux-kernel@...r.kernel.org <linux-kernel@...r.kernel.org>; xen-devel@...ts.xenproject.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 <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.

Sure, I will resend with an expanded commit message.

Ross