linux-kernel - Re: [RFC PATCH 4/4] iommu/vt-d: Identify domains using first level page table

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Message-ID: <20190925065006.GN28074@xz-x1>
Date:   Wed, 25 Sep 2019 14:50:06 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        kevin.tian@...el.com, Yi Sun <yi.y.sun@...ux.intel.com>,
        ashok.raj@...el.com, kvm@...r.kernel.org, sanjay.k.kumar@...el.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        yi.y.sun@...el.com
Subject: Re: [RFC PATCH 4/4] iommu/vt-d: Identify domains using first level
 page table

On Mon, Sep 23, 2019 at 08:24:54PM +0800, Lu Baolu wrote:
> +/*
> + * Check and return whether first level is used by default for
> + * DMA translation.
> + */
> +static bool first_level_by_default(void)
> +{
> +	struct dmar_drhd_unit *drhd;
> +	struct intel_iommu *iommu;
> +
> +	rcu_read_lock();
> +	for_each_active_iommu(iommu, drhd)
> +		if (!sm_supported(iommu) ||
> +		    !ecap_flts(iommu->ecap) ||
> +		    !cap_caching_mode(iommu->cap))
> +			return false;
> +	rcu_read_unlock();
> +
> +	return true;
> +}

"If no caching mode, then we will not use 1st level."

Hmm, does the vIOMMU needs to support caching-mode if with the
solution you proposed here?  Caching mode is only necessary for
shadowing AFAICT, and after all you're going to use full-nested,
then... then I would think it's not needed.  And if so, with this
patch 1st level will be disabled. Sounds like a paradox...

I'm thinking what would be the big picture for this to work now: For
the vIOMMU, instead of exposing the caching-mode, I'm thinking maybe
we should expose it with ecap.FLTS=1 while we can keep ecap.SLTS=0
then it's natural that we can only use 1st level translation in the
guest for all the domains (and I assume such an ecap value should
never happen on real hardware, am I right?).

Regards,

-- 
Peter Xu