linux-kernel - Re: [PATCH 1/3] x86/setup: always reserve the first 1M of RAM

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Message-ID: <YLZsEaimyAe0x6b3@kernel.org>
Date:   Tue, 1 Jun 2021 20:19:13 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Andy Shevchenko <andy@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Darren Hart <dvhart@...radead.org>,
        Dave Young <dyoung@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Lianbo Jiang <lijiang@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-doc@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH 1/3] x86/setup: always reserve the first 1M of RAM

Hi Baoquan,
On Tue, Jun 01, 2021 at 05:06:53PM +0800, Baoquan He wrote:
> On 06/01/21 at 10:53am, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@...ux.ibm.com>
> ......  
> 
> > diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
> > index 7850111008a8..b15ebfe40a73 100644
> > --- a/arch/x86/platform/efi/quirks.c
> > +++ b/arch/x86/platform/efi/quirks.c
> > @@ -450,6 +450,18 @@ void __init efi_free_boot_services(void)
> >  			size -= rm_size;
> >  		}
> 
> Thanks for taking care of the low-1M excluding in
> efi_free_boot_services(), Mike. You might want to remove the old real
> mode excluding code either since it's been covered by your new code.

Unfortunately I can't because it's important that set_real_mode_mem() would
reuse memory that was occupied by EFI boot services and that is being freed
here.

According to the changelog of 5bc653b73182 ("x86/efi: Allocate a trampoline
if needed in efi_free_boot_services()"), that system has EBDA at 0x2c000 so
we reserve everything from 0x2c000 to 0xa0000 in reserve_bios_regions() and
most of the memory below 0x2c0000 is used by EFI boot data. So with such
memory layout reserve_real_mode() won't be able to allocate the trampoline.
Yet, when the EFI boot data is free, the room occupied by it will be reused
by the real mode trampoline via set_real_mode_mem().
 
> diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
> index b15ebfe40a73..be814f2089ff 100644
> --- a/arch/x86/platform/efi/quirks.c
> +++ b/arch/x86/platform/efi/quirks.c
> @@ -409,7 +409,6 @@ void __init efi_free_boot_services(void)
>  	for_each_efi_memory_desc(md) {
>  		unsigned long long start = md->phys_addr;
>  		unsigned long long size = md->num_pages << EFI_PAGE_SHIFT;
> -		size_t rm_size;
>  
>  		if (md->type != EFI_BOOT_SERVICES_CODE &&
>  		    md->type != EFI_BOOT_SERVICES_DATA) {
> @@ -430,26 +429,6 @@ void __init efi_free_boot_services(void)
>  		 */
>  		efi_unmap_pages(md);
>  
> -		/*
> -		 * Nasty quirk: if all sub-1MB memory is used for boot
> -		 * services, we can get here without having allocated the
> -		 * real mode trampoline.  It's too late to hand boot services
> -		 * memory back to the memblock allocator, so instead
> -		 * try to manually allocate the trampoline if needed.
> -		 *
> -		 * I've seen this on a Dell XPS 13 9350 with firmware
> -		 * 1.4.4 with SGX enabled booting Linux via Fedora 24's
> -		 * grub2-efi on a hard disk.  (And no, I don't know why
> -		 * this happened, but Linux should still try to boot rather
> -		 * panicking early.)
> -		 */
> -		rm_size = real_mode_size_needed();
> -		if (rm_size && (start + rm_size) < (1<<20) && size >= rm_size) {
> -			set_real_mode_mem(start);
> -			start += rm_size;
> -			size -= rm_size;
> -		}
> -
>  		/*
>  		 * Don't free memory under 1M for two reasons:
>  		 * - BIOS might clobber it
> 
> >  
> > +		/*
> > +		 * Don't free memory under 1M for two reasons:
> > +		 * - BIOS might clobber it
> > +		 * - Crash kernel needs it to be reserved
> > +		 */
> > +		if (start + size < SZ_1M)
> > +			continue;
> > +		if (start < SZ_1M) {
> > +			size -= (SZ_1M - start);
> > +			start = SZ_1M;
> > +		}
> > +
> >  		memblock_free_late(start, size);
> >  	}
> >  
> > diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> > index 2e1c1bec0f9e..8ea285aca827 100644
> > --- a/arch/x86/realmode/init.c
> > +++ b/arch/x86/realmode/init.c
> > @@ -29,14 +29,16 @@ void __init reserve_real_mode(void)
> >  
> >  	/* Has to be under 1M so we can execute real-mode AP code. */
> >  	mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
> > -	if (!mem) {
> > +	if (!mem)
> >  		pr_info("No sub-1M memory is available for the trampoline\n");
> > -		return;
> > -	}
> > +	else
> > +		set_real_mode_mem(mem);
> >  
> > -	memblock_reserve(mem, size);
> > -	set_real_mode_mem(mem);
> > -	crash_reserve_low_1M();
> > +	/*
> > +	 * Unconditionally reserve the entire fisrt 1M, see comment in
> > +	 * setup_arch()
> > +	 */
> > +	memblock_reserve(0, SZ_1M);
> >  }
> >  
> >  static void sme_sev_setup_real_mode(struct trampoline_header *th)
> > -- 
> > 2.28.0
> > 
> 

-- 
Sincerely yours,
Mike.