[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v2 2/4] xen/arm: mpu: Define Xen start address for MPU systems
Hi Julien, On 24/09/2024 17:44, Julien Grall wrote: On 23/09/2024 12:22, Ayan Kumar Halder wrote:On 19/09/2024 13:42, Julien Grall wrote:Hi Ayan,Hi Julien,On 18/09/2024 19:51, Ayan Kumar Halder wrote:From: Wei Chen <wei.chen@xxxxxxx>On Armv8-A, Xen has a fixed virtual start address (link address too) for all Armv8-A platforms. In an MMU based system, Xen can map its loaded address to this virtual start address. So, on Armv8-A platforms, the Xen start address does not need to be configurable. But on Armv8-R platforms, there is no MMU to map loaded address to a fixed virtual address and different platforms will have very different address space layout. So Xen cannot use a fixed physical address onMPU based system and need to have it configurable.So, we introduce a Kconfig option for users to set the start address. The start address needs to be aligned to 4KB. We have a check for this alignment.It would suggest to add some explanation why you want the start address to be 4KB aligned.Let me know if this makes sense."MPU allows us to define regions which are 64 bits aligned. This restriction comes from the bitfields of PRBAR, PRLAR (the lower 6 bits are 0 extended to provide the base and limit address of a region). This means that the start address of Xen needs to be at least 64 bits aligned (as it will correspond to the start address of memory protection region).As for now Xen on MPU tries to use the same memory alignment restrictions as it has been for MMU. Unlike MMU where the starting virtual address is 2MB, Xen on MPU needs the start address to be 4 KB (ie page size) aligned."Thinking about this a bit more (and based on the discussion in "Re: [PATCH v1 2/4] xen/arm: mpu: Define Xen start address for MPU systems"), I think we can keep the 4 KB restriction for now and relax it later. Let me know what you think.I am fine with that.In case if the user forgets to set the start address, then 0xffffffff is used as default. This is to trigger the error (on alignment check) and thereby promptuser to set the start address.Also updated config.h so that it includes mpu/layout.h when CONFIG_MPU isdefined. Signed-off-by: Wei Chen <wei.chen@xxxxxxx> Signed-off-by: Jiamei.Xie <jiamei.xie@xxxxxxx> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx> --- Changes from :- v1 - 1. Fixed some of the coding style issues. 2. Reworded the help message. 3. Updated the commit message. xen/arch/arm/Kconfig | 10 ++++++++++ xen/arch/arm/include/asm/config.h | 4 +++-xen/arch/arm/include/asm/mpu/layout.h | 27 +++++++++++++++++++++++++++Looking at this patch again, I don't see any modification in xen.lds.S. Is it intended?If we agree with the justification provided before, then this should be the modification.--- a/xen/arch/arm/xen.lds.S +++ b/xen/arch/arm/xen.lds.S @@ -232,6 +232,12 @@ SECTIONS */ ASSERT(_start == XEN_VIRT_START, "_start != XEN_VIRT_START") +/*+ * On MPU based platforms, the starting address is to be provided by user.+ * One need to check that it is 4KB aligned. + */+ASSERT(IS_ALIGNED(_start, 4096), "starting address is misaligned")+Ah I wasn't asking to add an ASSERT (although it would be good so long it is protected by CONFIG_MPU). Instead I was expecting XEN_VIRT_START to be replaced. But looking at the code again, I see you defineXEN_VIRT_START for the MPU.I think this is a little bit odd to use XEN_VIRT_* defined for the MPU but it would be ok. Is the expectation that all the MPU code should be using XEN_START_ADDRESS? If so, maybe it would be worth add a comment on top of XEN_VIRT_ADDRESS. I have defined XEN_VIRT_START as XEN_START_ADDRESS , so that the code continues to use XEN_VIRT_START (as it does for the MMU) and we avoid introducing #if-defs. Does this comment make sense ? /* * For MPU, XEN's virtual start address is same as the physical address. * The reason being MPU treats VA == PA. IOW, it cannot map the physical * address to a different fixed virtual address. So, the virtual start * address is determined by the physical address at which Xen is loaded. */ #define XEN_VIRT_START _AT(paddr_t, XEN_START_ADDRESS) - Ayan Cheers,
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