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[Xen-devel] [PATCH v1 02/10] xen/pt: Sync up the dev.config and data values.

For a passthrough device we maintain a state of emulated
registers value contained within d->config. We also consult
the host registers (and apply ro and write masks) whenever
the guest access the registers. This is done in xen_pt_pci_write_config
and xen_pt_pci_read_config.

Also in this picture we call pci_default_write_config which
updates the d->config and if the d->config[PCI_COMMAND] register
has PCI_COMMAND_MEMORY (or PCI_COMMAND_IO) acts on those changes.

On startup the d->config[PCI_COMMAND] are the host values, not
what the guest initial values should be, which is exactly what
we do _not_ want to do for 64-bit BARs when the guest just wants
to read the size of the BAR. Huh you say?

To get the size of 64-bit memory space BARs,  the guest has
to calculate ((BAR[x] & 0xFFFFFFF0) + ((BAR[x+1] & 0xFFFFFFFF) << 32))
which means it has to do two writes of ~0 to BARx and BARx+1.

prior to this patch and with XSA120-addendum patch (Linux kernel)
the PCI_COMMAND register is copied from the host it can have
PCI_COMMAND_MEMORY bit set which means that QEMU will try to
update the hypervisor's P2M with BARx+1 value to ~0 (0xffffffff)
(to sync the guest state to host) instead of just having
xen_pt_pci_write_config and xen_pt_bar_reg_write apply the
proper masks and return the size to the guest.

To thwart this, this patch syncs up the host values with the
guest values taking into account the emu_mask (bit set means
we emulate, PCI_COMMAND_MEMORY and PCI_COMMAND_IO are set).
That is we copy the host values - masking out any bits which
we will emulate. Then merge it with the initial emulation register
values. Lastly this value is then copied both in
dev.config _and_ XenPTReg->data field.

There is also reg->size accounting taken into consideration
that ends up being used in two patches:
 xen/pt: Check if reg->init function sets the 'data' past the reg->size
 xen/pt: Use xen_host_pci_get_[byte,word,long] instead of xen_host_pci_get_long

This fixes errors such as these:

(XEN) memory_map:add: dom2 gfn=fffe0 mfn=fbce0 nr=20
(DEBUG) 189 pci dev 04:0 BAR16 wrote ~0.
(DEBUG) 200 pci dev 04:0 BAR16 read 0x0fffe0004.
(XEN) memory_map:remove: dom2 gfn=fffe0 mfn=fbce0 nr=20
(DEBUG) 204 pci dev 04:0 BAR16 wrote 0x0fffe0004.
(DEBUG) 217 pci dev 04:0 BAR16 read upper 0x000000000.
(XEN) memory_map:add: dom2 gfn=ffffffff00000 mfn=fbce0 nr=20
(XEN) p2m.c:883:d0v0 p2m_set_entry failed! mfn=ffffffffffffffff rc:-22
(XEN) memory_map:fail: dom2 gfn=ffffffff00000 mfn=fbce0 nr=20 ret:-22
(XEN) memory_map:remove: dom2 gfn=ffffffff00000 mfn=fbce0 nr=20
(XEN) p2m.c:920:d0v0 gfn_to_mfn failed! gfn=ffffffff00000 type:4
(XEN) p2m.c:920:d0v0 gfn_to_mfn failed! gfn=ffffffff00001 type:4
(XEN) memory_map: error -22 removing dom2 access to [fbce0,fbcff]
(DEBUG) 222 pci dev 04:0 BAR16 read upper 0x0ffffffff.
(XEN) memory_map:remove: dom2 gfn=ffffffff00000 mfn=fbce0 nr=20
(XEN) memory_map: error -22 removing dom2 access to [fbce0,fbcff]

[The DEBUG is to illustate what the hvmloader was doing]

Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
 hw/xen/xen_pt_config_init.c | 48 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/hw/xen/xen_pt_config_init.c b/hw/xen/xen_pt_config_init.c
index e34f9f8..3938afd 100644
--- a/hw/xen/xen_pt_config_init.c
+++ b/hw/xen/xen_pt_config_init.c
@@ -1856,6 +1856,10 @@ static int xen_pt_config_reg_init(XenPCIPassthroughState 
     reg_entry->reg = reg;
     if (reg->init) {
+        uint32_t host_mask, size_mask;
+        unsigned int offset;
+        uint32_t val;
         /* initialize emulate register */
         rc = reg->init(s, reg_entry->reg,
                        reg_grp->base_offset + reg->offset, &data);
@@ -1868,8 +1872,50 @@ static int xen_pt_config_reg_init(XenPCIPassthroughState 
             return 0;
+        /* Sync up the data to dev.config */
+        offset = reg_grp->base_offset + reg->offset;
+        size_mask = 0xFFFFFFFF >> ((4 - reg->size) << 3);
+        rc = xen_host_pci_get_long(&s->real_device, offset, &val);
+        if (rc) {
+            /* Serious issues when we cannot read the host values! */
+            g_free(reg_entry);
+            return rc;
+        }
+        /* Set bits in emu_mask are the ones we emulate. The dev.config shall
+         * contain the emulated view of the guest - therefore we flip the mask
+         * to mask out the host values (which dev.config initially has) . */
+        host_mask = size_mask & ~reg->emu_mask;
+        if ((data & host_mask) != (val & host_mask)) {
+            uint32_t new_val;
+            /* Mask out host (including past size). */
+            new_val = val & host_mask;
+            /* Merge emulated ones (excluding the non-emulated ones). */
+            new_val |= data & host_mask;
+            /* Leave intact host and emulated values past the size - even 
+             * we do not care as we write per reg->size granularity, but for 
+             * logging below lets have the proper value. */
+            new_val |= ((val | data)) & ~size_mask;
+            XEN_PT_LOG(&s->dev,"Offset 0x%04x mismatch! Emulated=0x%04x, 
host=0x%04x, syncing to 0x%04x.\n",
+                       offset, data, val, new_val);
+            val = new_val;
+        } else
+            val = data;
+        /* This could be just pci_set_long as we don't modify the bits
+         * past reg->size, but in case this routine is run in parallel
+         * we do not want to over-write other registers. */
+        switch (reg->size) {
+        case 1: pci_set_byte(s->dev.config + offset, (uint8_t)val); break;
+        case 2: pci_set_word(s->dev.config + offset, (uint16_t)val); break;
+        case 4: pci_set_long(s->dev.config + offset, val); break;
+        default: assert(1);
+        }
         /* set register value */
-        reg_entry->data = data;
+        reg_entry->data = val;
     /* list add register entry */
     QLIST_INSERT_HEAD(&reg_grp->reg_tbl_list, reg_entry, entries);

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