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Re: [PATCH v2 13/40] xen/mpu: introduce unified function setup_early_uart to map early UART
Hi Penny,
On 29/01/2023 06:17, Penny Zheng wrote:
-----Original Message-----
From: Julien Grall <julien@xxxxxxx>
Sent: Wednesday, January 25, 2023 3:09 AM
To: Penny Zheng <Penny.Zheng@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Wei Chen <Wei.Chen@xxxxxxx>; Stefano Stabellini
<sstabellini@xxxxxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>;
Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
Subject: Re: [PATCH v2 13/40] xen/mpu: introduce unified function
setup_early_uart to map early UART
Hi Peny,
Hi Julien,
On 13/01/2023 05:28, Penny Zheng wrote:
In MMU system, we map the UART in the fixmap (when earlyprintk is used).
However in MPU system, we map the UART with a transient MPU memory
region.
So we introduce a new unified function setup_early_uart to replace the
previous setup_fixmap.
Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
---
xen/arch/arm/arm64/head.S | 2 +-
xen/arch/arm/arm64/head_mmu.S | 4 +-
xen/arch/arm/arm64/head_mpu.S | 52
+++++++++++++++++++++++++
xen/arch/arm/include/asm/early_printk.h | 1 +
4 files changed, 56 insertions(+), 3 deletions(-)
diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 7f3f973468..a92883319d 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -272,7 +272,7 @@ primary_switched:
* afterwards.
*/
bl remove_identity_mapping
- bl setup_fixmap
+ bl setup_early_uart
#ifdef CONFIG_EARLY_PRINTK
/* Use a virtual address to access the UART. */
ldr x23, =EARLY_UART_VIRTUAL_ADDRESS
diff --git a/xen/arch/arm/arm64/head_mmu.S
b/xen/arch/arm/arm64/head_mmu.S index b59c40495f..a19b7c873d
100644
--- a/xen/arch/arm/arm64/head_mmu.S
+++ b/xen/arch/arm/arm64/head_mmu.S
@@ -312,7 +312,7 @@ ENDPROC(remove_identity_mapping)
*
* Clobbers x0 - x3
*/
-ENTRY(setup_fixmap)
+ENTRY(setup_early_uart)
This function is doing more than enable the early UART. It also setups the
fixmap even earlyprintk is not configured.
True, true.
I've thoroughly read the MMU implementation of setup_fixmap, and I'll try to
split
it up.
I am not entirely sure what could be the name. Maybe this needs to be split
further.
#ifdef CONFIG_EARLY_PRINTK
/* Add UART to the fixmap table */
ldr x0, =EARLY_UART_VIRTUAL_ADDRESS
@@ -325,7 +325,7 @@ ENTRY(setup_fixmap)
dsb nshst
ret
-ENDPROC(setup_fixmap)
+ENDPROC(setup_early_uart)
/* Fail-stop */
fail: PRINT("- Boot failed -\r\n")
diff --git a/xen/arch/arm/arm64/head_mpu.S
b/xen/arch/arm/arm64/head_mpu.S index e2ac69b0cc..72d1e0863d
100644
--- a/xen/arch/arm/arm64/head_mpu.S
+++ b/xen/arch/arm/arm64/head_mpu.S
@@ -18,8 +18,10 @@
#define REGION_TEXT_PRBAR 0x38 /* SH=11 AP=10 XN=00 */
#define REGION_RO_PRBAR 0x3A /* SH=11 AP=10 XN=10 */
#define REGION_DATA_PRBAR 0x32 /* SH=11 AP=00 XN=10 */
+#define REGION_DEVICE_PRBAR 0x22 /* SH=10 AP=00 XN=10 */
#define REGION_NORMAL_PRLAR 0x0f /* NS=0 ATTR=111 EN=1 */
+#define REGION_DEVICE_PRLAR 0x09 /* NS=0 ATTR=100 EN=1 */
/*
* Macro to round up the section address to be PAGE_SIZE aligned @@
-334,6 +336,56 @@ ENTRY(enable_mm)
ret
ENDPROC(enable_mm)
+/*
+ * Map the early UART with a new transient MPU memory region.
+ *
Missing "Inputs: "
+ * x27: region selector
+ * x28: prbar
+ * x29: prlar
+ *
+ * Clobbers x0 - x4
+ *
+ */
+ENTRY(setup_early_uart)
+#ifdef CONFIG_EARLY_PRINTK
+ /* stack LR as write_pr will be called later like nested function */
+ mov x3, lr
+
+ /*
+ * MPU region for early UART is a transient region, since it will be
+ * replaced by specific device memory layout when FDT gets parsed.
I would rather not mention "FDT" here because this code is independent to
the firmware table used.
However, any reason to use a transient region rather than the one that will
be used for the UART driver?
We don’t want to define a MPU region for each device driver. It will exhaust
MPU regions very quickly.
What the usual size of an MPU?
However, even if you don't want to define one for every device, it still
seem to be sensible to define a fixed temporary one for the early UART
as this would simplify the assembly code.
In commit " [PATCH v2 28/40] xen/mpu: map boot module section in MPU system",
Did you mean patch #27?
A new FDT property `mpu,device-memory-section` will be introduced for users to
statically
configure the whole system device memory with the least number of memory
regions in Device Tree.
This section shall cover all devices that will be used in Xen, like `UART`,
`GIC`, etc.
For FVP_BaseR_AEMv8R, we have the following definition:
```
mpu,device-memory-section = <0x0 0x80000000 0x0 0x7ffff000>;
```
I am a bit worry this will be a recipe for mistake. Do you have an
example where the MPU will be exhausted if we reserve some entries for
each device (or some)?
Cheers,
--
Julien Grall
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