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

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

> From: Dave Hansen <dave.hansen@xxxxxxxxx>
> Sent: Thursday, June 1, 2023 5:57 PM
> To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx 
> <linux-kernel@xxxxxxxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx 
> <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>; 
> Ingo Molnar <mingo@xxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Dave Hansen 
> <dave.hansen@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx <x86@xxxxxxxxxx>; Juergen Gross 
> <jgross@xxxxxxxx>; Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>; Peter Jones 
> <pjones@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>
> 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