Re: [Qemu-devel] [PATCH v3 1/2] intel-iommu: differentiate host address width from IOVA address width.

[Eduardo Habkost]

On Wed, Dec 19, 2018 at 11:40:37AM +0100, Igor Mammedov wrote:
> On Wed, 19 Dec 2018 10:57:17 +0800
> Yu Zhang <address@hidden> wrote:
> 
> > On Tue, Dec 18, 2018 at 03:55:36PM +0100, Igor Mammedov wrote:
> > > On Tue, 18 Dec 2018 17:27:23 +0800
> > > Yu Zhang <address@hidden> wrote:
> > >   
> > > > On Mon, Dec 17, 2018 at 02:17:40PM +0100, Igor Mammedov wrote:  
> > > > > On Wed, 12 Dec 2018 21:05:38 +0800
> > > > > Yu Zhang <address@hidden> wrote:
> > > > >   
> > > > > > Currently, vIOMMU is using the value of IOVA address width, instead 
> > > > > > of
> > > > > > the host address width(HAW) to calculate the number of reserved 
> > > > > > bits in
> > > > > > data structures such as root entries, context entries, and entries 
> > > > > > of
> > > > > > DMA paging structures etc.
> > > > > > 
> > > > > > However values of IOVA address width and of the HAW may not equal. 
> > > > > > For
> > > > > > example, a 48-bit IOVA can only be mapped to host addresses no 
> > > > > > wider than
> > > > > > 46 bits. Using 48, instead of 46 to calculate the reserved bit may 
> > > > > > result
> > > > > > in an invalid IOVA being accepted.
> > > > > > 
> > > > > > To fix this, a new field - haw_bits is introduced in struct 
> > > > > > IntelIOMMUState,
> > > > > > whose value is initialized based on the maximum physical address 
> > > > > > set to
> > > > > > guest CPU.  
> > > > >   
> > > > > > Also, definitions such as VTD_HOST_AW_39/48BIT etc. are renamed
> > > > > > to clarify.
> > > > > > 
> > > > > > Signed-off-by: Yu Zhang <address@hidden>
> > > > > > Reviewed-by: Peter Xu <address@hidden>
> > > > > > ---  
> > > > > [...]
> > > > >   
> > > > > > @@ -3100,6 +3104,8 @@ static void 
> > > > > > vtd_iommu_replay(IOMMUMemoryRegion *iommu_mr, IOMMUNotifier *n)
> > > > > >  static void vtd_init(IntelIOMMUState *s)
> > > > > >  {
> > > > > >      X86IOMMUState *x86_iommu = X86_IOMMU_DEVICE(s);
> > > > > > +    CPUState *cs = first_cpu;
> > > > > > +    X86CPU *cpu = X86_CPU(cs);
> > > > > >  
> > > > > >      memset(s->csr, 0, DMAR_REG_SIZE);
> > > > > >      memset(s->wmask, 0, DMAR_REG_SIZE);
> > > > > > @@ -3119,23 +3125,24 @@ static void vtd_init(IntelIOMMUState *s)
> > > > > >      s->cap = VTD_CAP_FRO | VTD_CAP_NFR | VTD_CAP_ND |
> > > > > >               VTD_CAP_MAMV | VTD_CAP_PSI | VTD_CAP_SLLPS |
> > > > > >               VTD_CAP_SAGAW_39bit | VTD_CAP_MGAW(s->aw_bits);
> > > > > > -    if (s->aw_bits == VTD_HOST_AW_48BIT) {
> > > > > > +    if (s->aw_bits == VTD_AW_48BIT) {
> > > > > >          s->cap |= VTD_CAP_SAGAW_48bit;
> > > > > >      }
> > > > > >      s->ecap = VTD_ECAP_QI | VTD_ECAP_IRO;
> > > > > > +    s->haw_bits = cpu->phys_bits;  
> > > > > Is it possible to avoid accessing CPU fields directly or cpu 
> > > > > altogether
> > > > > and set phys_bits when iommu is created?  
> > > > 
> > > > Thanks for your comments, Igor.
> > > > 
> > > > Well, I guess you prefer not to query the CPU capabilities while 
> > > > deciding
> > > > the vIOMMU features. But to me, they are not that irrelevant.:)
> > > > 
> > > > Here the hardware address width in vt-d, and the one in 
> > > > cpuid.MAXPHYSADDR
> > > > are referring to the same concept. In VM, both are the maximum guest 
> > > > physical
> > > > address width. If we do not check the CPU field here, we will still 
> > > > have to
> > > > check the CPU field in other places such as build_dmar_q35(), and reset 
> > > > the
> > > > s->haw_bits again.
> > > > 
> > > > Is this explanation convincing enough? :)  
> > > current build_dmar_q35() doesn't do it, it's all new code in this series 
> > > that
> > > contains not acceptable direct access from one device (iommu) to another 
> > > (cpu).   
> > > Proper way would be for the owner of iommu to fish limits from somewhere 
> > > and set
> > > values during iommu creation.  
> > 
> > Well, current build_dmar_q35() doesn't do it, because it is using the 
> > incorrect value. :)
> > According to the spec, the host address width is the maximum physical 
> > address width,
> > yet current implementation is using the DMA address width. For me, this is 
> > not only
> > wrong, but also unsecure. For this point, I think we all agree this need to 
> > be fixed.
> > 
> > As to how to fix it - should we query the cpu fields, I still do not 
> > understand why
> > this is not acceptable. :)
> > 
> > I had thought of other approaches before, yet I did not choose:
> > 
> > 1> Introduce a new parameter, say, "x-haw-bits" which is used for iommu to 
> > limit its  
> > physical address width(similar to the "x-aw-bits" for IOVA). But what 
> > should we check
> > this parameter or not? What if this parameter is set to sth. different than 
> > the "phys-bits"
> > or not?
> > 
> > 2> Another choice I had thought of is, to query the physical iommu. I 
> > abandoned this  
> > idea because my understanding is that vIOMMU is not a passthrued device, it 
> > is emulated.
> 
> > So Igor, may I ask why you think checking against the cpu fields so not 
> > acceptable? :)
> Because accessing private fields of device from another random device is not 
> robust
> and a subject to breaking in unpredictable manner when field meaning or 
> initialization
> order changes. (analogy to baremetal: one does not solder wire to a CPU die 
> to let
> access some piece of data from random device).
> 

With either the solution below or the one I proposed, we still
have a ordering problem: if we want "-cpu ...,phys-bits=..." to
affect the IOMMU device, we will need the CPU objects to be
created before IOMMU realize.

At least both proposals make the initialization ordering
explicitly a responsibility of the machine code.  In either case,
I don't think we will start creating all CPU objects after device
realize any time soon.


> I've looked at intel-iommu code and how it's created so here is a way to do 
> the thing
> you need using proper interfaces:
> 
> 1. add x-haw_bits property
> 2. include in your series patch
>     '[Qemu-devel] [PATCH] qdev: let machine hotplug handler to override  bus 
> hotplug handler'
> 3. add your iommu to pc_get_hotpug_handler() to redirect plug flow to
>    machine and let _pre_plug handler to check and set x-haw_bits for machine 
> level

Wow, that's a very complex way to pass a single integer from
machine code to device code.  If this is the only way to do that,
we really need to take a step back and rethink our API design.

What's wrong with having a simple
  uint32_t pc_max_phys_bits(PCMachineState*)
function?

> 4. you probably can use phys-bits/host-phys-bits properties to get data that 
> you need
>    also see how ms->possible_cpus, that's how you can get access to CPU from 
> machine
>    layer.
> 
[...]

-- 
Eduardo