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Re: [Qemu-devel] [PATCH 7/7] intel_iommu: support passthrough (PT)

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH 7/7] intel_iommu: support passthrough (PT)
Date: Thu, 20 Apr 2017 13:18:28 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 2017年04月18日 12:21, Peter Xu wrote:
On Tue, Apr 18, 2017 at 12:00:13PM +0800, Jason Wang wrote:

On 2017年04月18日 11:50, Peter Xu wrote:
On Tue, Apr 18, 2017 at 11:23:35AM +0800, Jason Wang wrote:
On 2017年04月17日 18:58, Peter Xu wrote:

+static void vtd_switch_address_space(VTDAddressSpace *as)
+    bool use_iommu;
+    assert(as);
+    use_iommu = as->iommu_state->dmar_enabled;
+    if (use_iommu) {
+        /* Further checks per-device configuration */
+        use_iommu &= !vtd_dev_pt_enabled(as);
+    }
Looks like you can use as->iommu_state->dmar_enabled &&
vtd_dev_pt_enalbed() needs to read the guest memory (starting from
reading root entry), which is slightly slow. I was trying to avoid
unecessary reads.

I think compiler won't go to vtd_dev_pt_enabled() if dmar_enabled is false.
You are right. I'll switch.

@@ -991,6 +1058,18 @@ static void vtd_do_iommu_translate(VTDAddressSpace 
*vtd_as, PCIBus *bus,
          cc_entry->context_cache_gen = s->context_cache_gen;
+    /*
+     * We don't need to translate for pass-through context entries.
+     * Also, let's ignore IOTLB caching as well for PT devices.
+     */
+    if (vtd_ce_get_type(&ce) == VTD_CONTEXT_TT_PASS_THROUGH) {
+        entry->translated_addr = entry->iova;
+        entry->addr_mask = VTD_PAGE_SIZE - 1;
+        entry->perm = IOMMU_RW;
+        trace_vtd_translate_pt(source_id, entry->iova);
+        return;
+    }
Several questions here:

1) Is this just for vhost?
No. When caching mode is not enabled, all passthroughed devices should
be using this path.
Ok, then it looks better to switch the address space if we've found it was
Do you mean to switch in that if() above? Then when invalidate context
entry, we switch back if needed?


2) Since this is done after IOTLB querying, do we need flush IOTLB during
address switching?
IMHO if guest switches address space for a device, it is required to
send IOTLB flush as well for that device/domain.


  static void vtd_switch_address_space_all(IntelIOMMUState *s)
      GHashTableIter iter;
@@ -2849,6 +2914,10 @@ static void vtd_init(IntelIOMMUState *s)
          s->ecap |= VTD_ECAP_DT;
+    if (x86_iommu->pt_supported) {
+        s->ecap |= VTD_ECAP_PT;
+    }
Since we support migration now, need compat this for pre 2.10.
Oh yes. If I set pt=off by default, it should be okay then, right?
Right, but I think it's better to keep this on by default for performance
Okay. Just to confirm, that'll need one entry for HW_COMPAT_2_9,
right? (though it is still not there)



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