linux-kernel - Re: [PATCH] x86, efi: 1:1 pagetable mapping for virtual EFI calls

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Message-Id: <5048D09A020000780009965B@nat28.tlf.novell.com>
Date:	Thu, 06 Sep 2012 15:34:34 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Matt Fleming" <matt@...sole-pimps.org>
Cc:	"Vasco Dias" <rafa.vasco@...il.com>,
	"Matt Fleming" <matt.fleming@...el.com>,
	"Matthew Garrett" <mjg@...hat.com>, <linux-efi@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <cJ-ko@...gloub.eu>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86, efi: 1:1 pagetable mapping for virtual EFI
 calls

>>> On 06.09.12 at 15:15, Matt Fleming <matt@...sole-pimps.org> wrote:
> From: Matt Fleming <matt.fleming@...el.com>
> 
> Some firmware still needs a 1:1 (virt->phys) mapping even after we've
> called SetVirtualAddressMap(). So install the mapping alongside our
> existing kernel mapping whenever we make EFI calls in virtual mode.
> 
> This bug was discovered on ASUS machines where the firmware
> implementation of GetTime() accesses the RTC device via physical
> addresses, even though that's bogus per the UEFI spec since we've
> informed the firmware via SetVirtualAddressMap() that the boottime
> memory map is no longer valid.

> @@ -741,6 +742,114 @@ static void __init runtime_code_page_mkexec(void)
>  	}
>  }
>  
> +#ifdef CONFIG_X86_64
> +pgd_t *efi_one_to_one_pgd;
> +int efi_one_to_one_index = 0;

I wasn't able to spot where this variable ever gets set to a
non-zero value.

> @@ -58,6 +62,47 @@ static void __init early_code_mapping_set_exec(int 
> executable)
>  	}
>  }
>  
> +pgd_t *efi_call_virt_prelog(void)
> +{
> +	pgd_t *save;
> +	int i;
> +
> +	save = kmalloc(sizeof(pgd_t) * (efi_one_to_one_index + 1), GFP_KERNEL);

GFP_ATOMIC perhaps, given that many runtime calls happen
inside spin locks?

> +	if (!save) {
> +		pr_alert("Unable to save pgd entries\n");
> +		return NULL;
> +	}
> +
> +	for (i = 0; i <= efi_one_to_one_index; i++) {
> +		pgd_t *pgd = __va(read_cr3() & PHYSICAL_PAGE_MASK);

I'd suggest moving this out of the loop (also below), slightly
adjusting the rest of the loop body.

> +
> +		pgd += i;
> +		save[i] = *pgd;
> +		set_pgd(pgd, efi_one_to_one_pgd[i]);
> +	}

Did you, as an alternative, consider switching to a different
CR3 instead of copying back and forth?

> +
> +	__flush_tlb_all();

Is it certain you will _never_ hit a global mapping (in which case I
believe the above would be insufficient)?

Jan

> +	return save;
> +}
> +
> +void efi_call_virt_epilog(pgd_t *save)
> +{
> +	int i;
> +
> +	if (!save)
> +		return;
> +
> +	for (i = 0; i <= efi_one_to_one_index; i++) {
> +		pgd_t *pgd = __va(read_cr3() & PHYSICAL_PAGE_MASK);
> +
> +		pgd += i;
> +		set_pgd(pgd, save[i]);
> +	}
> +
> +	kfree(save);
> +	__flush_tlb_all();
> +}
> +
>  void __init efi_call_phys_prelog(void)
>  {
>  	unsigned long vaddress;
> -- 
> 1.7.4.4



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