linux-kernel - Re: [PATCH 2/2] iommu: Use the user PGD for SVA if PTI is enabled

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Date:   Mon, 22 Aug 2022 15:31:20 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, iommu@...ts.linux.dev,
        x86@...nel.org, Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Raj Ashok <ashok.raj@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>
Subject: Re: [PATCH 2/2] iommu: Use the user PGD for SVA if PTI is enabled

On 8/22/22 13:12, Jacob Pan wrote:
> @@ -394,7 +395,9 @@ static struct iommu_sva *intel_svm_bind_mm(struct intel_iommu *iommu,
>  	sflags = (flags & SVM_FLAG_SUPERVISOR_MODE) ?
>  			PASID_FLAG_SUPERVISOR_MODE : 0;
>  	sflags |= cpu_feature_enabled(X86_FEATURE_LA57) ? PASID_FLAG_FL5LP : 0;
> -	ret = intel_pasid_setup_first_level(iommu, dev, mm->pgd, mm->pasid,
> +
> +	pgd = static_cpu_has(X86_FEATURE_PTI) ? kernel_to_user_pgdp(mm->pgd) : mm->pgd;
> +	ret = intel_pasid_setup_first_level(iommu, dev, pgd, mm->pasid,
>  					    FLPT_DEFAULT_DID, sflags);

This X86_FEATURE_PTI should really be done within a helper.

I'd probably do this with a *new* helper since all of the existing
kernel_to_user_pgdp() users seem to be within a PTI #ifdef.

Maybe something like:

pgd_t *mm_user_pgd(struct mm_struct *mm)
{
#ifdef CONFIG_PAGE_TABLE_ISOLATION
	if (cpu_feature_enabled(X86_FEATURE_PTI))
		return kernel_to_user_pgdp(mm->pgd);
#endif
	return mm->pgd;
}

That #ifdef could even go away if your kernel_to_user_pgdp() stub from
patch 1/2 was available.  I'm not sure it's worth it though.