lguest: support backdoor window.

The VIRTIO_PCI_CAP_PCI_CFG in the PCI virtio 1.0 spec allows access to
the BAR registers without mapping them.  This is a compulsory feature,
and we implement it here.

There are some subtleties involving access widths which we should
note:

4.1.4.7.1 Device Requirements: PCI configuration access capability

...
   Upon detecting driver write access to pci_cfg_data, the device MUST
   execute a write access at offset cap.offset at BAR selected by
   cap.bar using the first cap.length bytes from pci_cfg_data.

   Upon detecting driver read access to pci_cfg_data, the device MUST
   execute a read access of length cap.length at offset cap.offset at
   BAR selected by cap.bar and store the first cap.length bytes in
   pci_cfg_data.

So, for a write, we copy into the pci_cfg_data window, then write from
there out to the BAR.  This works correctly if cap.length != width of
write.  Similarly, for a read, we read into window from the BAR then
read the value from there.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 100 insertions, 1 deletions

diff --git a/tools/lguest/lguest.c b/tools/lguest/lguest.c
index 8959ac246668..e3c4d3d7dc2a 100644
--- a/tools/lguest/lguest.c
+++ b/tools/lguest/lguest.c
@@ -156,7 +156,6 @@ struct pci_config {
         struct virtio_pci_notify_cap notify;
         struct virtio_pci_cap isr;
         struct virtio_pci_cap device;
-        /* FIXME: Implement this! */
         struct virtio_pci_cfg_cap cfg_access;
 };
 
@@ -1184,6 +1183,36 @@ static struct device *dev_and_reg(u32 *reg)
         return find_pci_device(pci_config_addr.bits.devnum);
 }
 
+/*
+ * We can get invalid combinations of values while they're writing, so we
+ * only fault if they try to write with some invalid bar/offset/length.
+ */
+static bool valid_bar_access(struct device *d,
+                             struct virtio_pci_cfg_cap *cfg_access)
+{
+        /* We only have 1 bar (BAR0) */
+        if (cfg_access->cap.bar != 0)
+                return false;
+
+        /* Check it's within BAR0. */
+        if (cfg_access->cap.offset >= d->mmio_size
+            || cfg_access->cap.offset + cfg_access->cap.length > d->mmio_size)
+                return false;
+
+        /* Check length is 1, 2 or 4. */
+        if (cfg_access->cap.length != 1
+            && cfg_access->cap.length != 2
+            && cfg_access->cap.length != 4)
+                return false;
+
+        /* Offset must be multiple of length */
+        if (cfg_access->cap.offset % cfg_access->cap.length != 0)
+                return false;
+
+        /* Return pointer into word in BAR0. */
+        return true;
+}
+
 /* Is this accessing the PCI config address port?. */
 static bool is_pci_addr_port(u16 port)
 {
@@ -1215,6 +1244,8 @@ static bool is_pci_data_port(u16 port)
         return port >= PCI_CONFIG_DATA && port < PCI_CONFIG_DATA + 4;
 }
 
+static void emulate_mmio_write(struct device *d, u32 off, u32 val, u32 mask);
+
 static bool pci_data_iowrite(u16 port, u32 mask, u32 val)
 {
         u32 reg, portoff;
@@ -1255,12 +1286,53 @@ static bool pci_data_iowrite(u16 port, u32 mask, u32 val)
                    && mask == 0xFFFF) {
                 /* Ignore command writes. */
                 return true;
+        } else if (&d->config_words[reg]
+                   == (void *)&d->config.cfg_access.cap.bar
+                   || &d->config_words[reg]
+                   == &d->config.cfg_access.cap.length
+                   || &d->config_words[reg]
+                   == &d->config.cfg_access.cap.offset) {
+
+                /*
+                 * The VIRTIO_PCI_CAP_PCI_CFG capability
+                 * provides a backdoor to access the MMIO
+                 * regions without mapping them.  Weird, but
+                 * useful.
+                 */
+                iowrite(portoff, val, mask, &d->config_words[reg]);
+                return true;
+        } else if (&d->config_words[reg] == &d->config.cfg_access.window) {
+                u32 write_mask;
+
+                /* Must be bar 0 */
+                if (!valid_bar_access(d, &d->config.cfg_access))
+                        return false;
+
+                /* First copy what they wrote into the window */
+                iowrite(portoff, val, mask, &d->config.cfg_access.window);
+
+                /*
+                 * Now emulate a write.  The mask we use is set by
+                 * len, *not* this write!
+                 */
+                write_mask = (1ULL<<(8*d->config.cfg_access.cap.length)) - 1;
+                verbose("Window writing %#x/%#x to bar %u, offset %u len %u\n",
+                        d->config.cfg_access.window, write_mask,
+                        d->config.cfg_access.cap.bar,
+                        d->config.cfg_access.cap.offset,
+                        d->config.cfg_access.cap.length);
+
+                emulate_mmio_write(d, d->config.cfg_access.cap.offset,
+                                   d->config.cfg_access.window, write_mask);
+                return true;
         }
 
         /* Complain about other writes. */
         return false;
 }
 
+static u32 emulate_mmio_read(struct device *d, u32 off, u32 mask);
+
 static void pci_data_ioread(u16 port, u32 mask, u32 *val)
 {
         u32 reg;
@@ -1268,6 +1340,33 @@ static void pci_data_ioread(u16 port, u32 mask, u32 *val)
 
         if (!d)
                 return;
+
+        /* Read through the PCI MMIO access window is special */
+        if (&d->config_words[reg] == &d->config.cfg_access.window) {
+                u32 read_mask;
+
+                /* Must be bar 0 */
+                if (!valid_bar_access(d, &d->config.cfg_access))
+                        errx(1, "Invalid cfg_access to bar%u, offset %u len %u",
+                             d->config.cfg_access.cap.bar,
+                             d->config.cfg_access.cap.offset,
+                             d->config.cfg_access.cap.length);
+
+                /*
+                 * Read into the window.  The mask we use is set by
+                 * len, *not* this read!
+                 */
+                read_mask = (1ULL<<(8*d->config.cfg_access.cap.length))-1;
+                d->config.cfg_access.window
+                        = emulate_mmio_read(d,
+                                            d->config.cfg_access.cap.offset,
+                                            read_mask);
+                verbose("Window read %#x/%#x from bar %u, offset %u len %u\n",
+                        d->config.cfg_access.window, read_mask,
+                        d->config.cfg_access.cap.bar,
+                        d->config.cfg_access.cap.offset,
+                        d->config.cfg_access.cap.length);
+        }
         ioread(port - PCI_CONFIG_DATA, d->config_words[reg], mask, val);
 }