linux-kernel - Re: [PATCH] iommu/vt-d: remove redundant variable no_platform

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Date:   Thu, 5 Nov 2020 09:40:37 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Zhenzhong Duan <zhenzhong.duan@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     baolu.lu@...ux.intel.com, dwmw2@...radead.org, joro@...tes.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] iommu/vt-d: remove redundant variable no_platform_optin

Hi Zhenzhong,

On 11/4/20 4:19 PM, Zhenzhong Duan wrote:
> no_platform_optin is redundant with dmar_disabled and it's only used in
> platform_optin_force_iommu(), remove it and use dmar_disabled instead.

It's actually not.

If CONFIG_INTEL_IOMMU_DEFAULT_ON is not set, we will get "dmar_disable =
1" and "no_platform_optin = 0". In this case, we must force the iommu on
and set dmar_disable = 0.

The real use case: if a kernel built with [CONFIG_INTEL_IOMMU_DEFAULT_ON
= n] running on a platform with thunderbolt ports, we must force IOMMU
on so that the system could be protected from possible malicious
peripherals.

Best regards,
baolu


> 
> Meanwhile remove all the dead code in platform_optin_force_iommu().
> 
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...il.com>
> ---
>   drivers/iommu/intel/iommu.c | 14 ++------------
>   1 file changed, 2 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 8651f6d4dfa0..a011d1ed63ef 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -180,7 +180,6 @@ static int rwbf_quirk;
>    */
>   static int force_on = 0;
>   int intel_iommu_tboot_noforce;
> -static int no_platform_optin;
>   
>   #define ROOT_ENTRY_NR (VTD_PAGE_SIZE/sizeof(struct root_entry))
>   
> @@ -440,7 +439,6 @@ static int __init intel_iommu_setup(char *str)
>   			pr_info("IOMMU enabled\n");
>   		} else if (!strncmp(str, "off", 3)) {
>   			dmar_disabled = 1;
> -			no_platform_optin = 1;
>   			pr_info("IOMMU disabled\n");
>   		} else if (!strncmp(str, "igfx_off", 8)) {
>   			dmar_map_gfx = 0;
> @@ -4810,20 +4808,12 @@ static inline bool has_external_pci(void)
>   
>   static int __init platform_optin_force_iommu(void)
>   {
> -	if (!dmar_platform_optin() || no_platform_optin || !has_external_pci())
> +	if (!dmar_platform_optin() || dmar_disabled || !has_external_pci())
>   		return 0;
>   
> -	if (no_iommu || dmar_disabled)
> +	if (no_iommu)
>   		pr_info("Intel-IOMMU force enabled due to platform opt in\n");
>   
> -	/*
> -	 * If Intel-IOMMU is disabled by default, we will apply identity
> -	 * map for all devices except those marked as being untrusted.
> -	 */
> -	if (dmar_disabled)
> -		iommu_set_default_passthrough(false);
> -
> -	dmar_disabled = 0;
>   	no_iommu = 0;
>   
>   	return 1;
>