linux-kernel - Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

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Message-ID: <CAGsJ_4w+ed78cnJusM_enEZpdGghzvjgt0aYDPpfwk4z7PQqxQ@mail.gmail.com>
Date:   Thu, 7 Oct 2021 18:43:33 +1300
From:   Barry Song <21cnbao@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>, mike.campin@...el.com,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

On Tue, Oct 5, 2021 at 7:21 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Mon, Oct 04, 2021 at 09:40:03AM -0700, Jacob Pan wrote:
> > Hi Barry,
> >
> > On Sat, 2 Oct 2021 01:45:59 +1300, Barry Song <21cnbao@...il.com> wrote:
> >
> > > >
> > > > > I assume KVA mode can avoid this iotlb flush as the device is using
> > > > > the page table of the kernel and sharing the whole kernel space. But
> > > > > will users be glad to accept this mode?
> > > >
> > > > You can avoid the lock be identity mapping the physical address space
> > > > of the kernel and maping map/unmap a NOP.
> > > >
> > > > KVA is just a different way to achive this identity map with slightly
> > > > different security properties than the normal way, but it doesn't
> > > > reach to the same security level as proper map/unmap.
> > > >
> > > > I'm not sure anyone who cares about DMA security would see value in
> > > > the slight difference between KVA and a normal identity map.
> > >
> > > yes. This is an important question. if users want a high security level,
> > > kva might not their choice; if users don't want the security, they are
> > > using iommu passthrough. So when will users choose KVA?
> > Right, KVAs sit in the middle in terms of performance and security.
> > Performance is better than IOVA due to IOTLB flush as you mentioned. Also
> > not too far behind of pass-through.
>
> The IOTLB flush is not on a DMA path but on a vmap path, so it is very
> hard to compare the two things.. Maybe vmap can be made to do lazy
> IOTLB flush or something and it could be closer
>
> > Security-wise, KVA respects kernel mapping. So permissions are better
> > enforced than pass-through and identity mapping.
>
> Is this meaningful? Isn't the entire physical map still in the KVA and
> isn't it entirely RW ?

Some areas are RX, for example, ARCH64 supports KERNEL_TEXT_RDONLY.
But the difference is really minor.

So do we have a case where devices can directly access the kernel's data
structure such as a list/graph/tree with pointers to a kernel virtual address?
then devices don't need to translate the address of pointers in a structure.
I assume this is one of the most useful features userspace SVA can provide.

But do we have a case where accelerators/GPU want to use the complex data
structures of kernel drivers?

>
> Jason

Thanks
barry