linux-kernel - Re: [PATCH 5.0 30/46] x86/boot/compressed/64: Do not read legacy ROM on EFI system

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Date:   Tue, 12 Mar 2019 12:50:26 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     hpa@...or.com
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Pitam Mitra <pitamm@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bockjoo Kim <bockjoo@...s.ufl.edu>, bp@...en8.de
Subject: Re: [PATCH 5.0 30/46] x86/boot/compressed/64: Do not read legacy ROM
 on EFI system

On Sat, Mar 09, 2019 at 10:10:19PM -0800, hpa@...or.com wrote:
> >@@ -47,8 +50,18 @@ static unsigned long find_trampoline_pla
> > 	 * This code is based on reserve_bios_regions().
> > 	 */
> > 
> >-	ebda_start = *(unsigned short *)0x40e << 4;
> >-	bios_start = *(unsigned short *)0x413 << 10;
> >+	/*
> >+	 * EFI systems may not provide legacy ROM. The memory may not be
> >mapped
> >+	 * at all.
> >+	 *
> >+	 * Only look for values in the legacy ROM for non-EFI system.
> >+	 */
> >+	signature = (char *)&boot_params->efi_info.efi_loader_signature;
> >+	if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
> >+	    strncmp(signature, EFI64_LOADER_SIGNATURE, 4)) {
> >+		ebda_start = *(unsigned short *)0x40e << 4;
> >+		bios_start = *(unsigned short *)0x413 << 10;
> >+	}
> > 
> > 	if (bios_start < BIOS_START_MIN || bios_start > BIOS_START_MAX)
> > 		bios_start = BIOS_START_MAX;
> 
> Only one objection: the explanation is nonsensical. 

Well, that's the best explanation I've come up with :/

I would be glad for any insight here.

-- 
 Kirill A. Shutemov