Re: [PATCH v2] physmem: allow cpu_memory_rw_debug to write to MMIO devices

[David Hildenbrand]

On 08.01.25 19:35, Stefan Zabka wrote:
> On 21/12/2024 15:55, David Hildenbrand wrote:
>   > Let's wait for opinions from others first.
>
> <https://www.qemu.org/docs/master/devel/submitting-a-patch.html#if-your-patch-seems-to-have-been-ignored>
> states that two weeks is a reasonable amount of time for follow-up.
>
> Should I also ping the original patch? I thought pinging the thread
> would be more appropriate, as it contains relevant information.

I just pushed a compiling version of the attrs.debug approach to:

        https://github.com/davidhildenbrand/qemu/tree/debug_access

With two preparation patches, the relevant patch is:


From 2e85cb1724385e4b8640364415832c030e5c5e6d Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Wed, 8 Jan 2025 20:58:00 +0100
Subject: [PATCH] physmem: allow cpu_memory_rw_debug to write to MMIO devices

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 hw/core/cpu-sysemu.c    | 13 +++++++++----
 include/exec/memattrs.h |  5 ++++-
 include/exec/memory.h   |  2 ++
 system/physmem.c        |  9 ++-------
 4 files changed, 17 insertions(+), 12 deletions(-)

diff --git a/hw/core/cpu-sysemu.c b/hw/core/cpu-sysemu.c
index 2a9a2a4eb5..0aa0a569e4 100644
--- a/hw/core/cpu-sysemu.c
+++ b/hw/core/cpu-sysemu.c
@@ -51,13 +51,18 @@ hwaddr cpu_get_phys_page_attrs_debug(CPUState *cpu, vaddr 
addr,
                                      MemTxAttrs *attrs)
 {
     CPUClass *cc = CPU_GET_CLASS(cpu);
+    hwaddr paddr;
 
     if (cc->sysemu_ops->get_phys_page_attrs_debug) {
-        return cc->sysemu_ops->get_phys_page_attrs_debug(cpu, addr, attrs);
+        paddr = cc->sysemu_ops->get_phys_page_attrs_debug(cpu, addr, attrs);
+    } else {
+        /* Fallback for CPUs which don't implement the _attrs_ hook */
+        *attrs = MEMTXATTRS_UNSPECIFIED;
+        paddr = cc->sysemu_ops->get_phys_page_debug(cpu, addr);
     }
-    /* Fallback for CPUs which don't implement the _attrs_ hook */
-    *attrs = MEMTXATTRS_UNSPECIFIED;
-    return cc->sysemu_ops->get_phys_page_debug(cpu, addr);
+    /* Indicate that this is a debug access. */
+    attrs->debug = 1;
+    return paddr;
 }
 
 hwaddr cpu_get_phys_page_debug(CPUState *cpu, vaddr addr)
diff --git a/include/exec/memattrs.h b/include/exec/memattrs.h
index e27c18f3dc..14e0edaa58 100644
--- a/include/exec/memattrs.h
+++ b/include/exec/memattrs.h
@@ -26,7 +26,8 @@ typedef struct MemTxAttrs {
     /* Bus masters which don't specify any attributes will get this
      * (via the MEMTXATTRS_UNSPECIFIED constant), so that we can
      * distinguish "all attributes deliberately clear" from
-     * "didn't specify" if necessary.
+     * "didn't specify" if necessary. "debug" can be set alongside
+     * "unspecified".
      */
     unsigned int unspecified:1;
     /*
@@ -50,6 +51,8 @@ typedef struct MemTxAttrs {
      * (see MEMTX_ACCESS_ERROR).
      */
     unsigned int memory:1;
+    /* Debug access that can even write to ROM. */
+    unsigned int debug:1;
     /* Requester ID (for MSI for example) */
     unsigned int requester_id:16;
 
diff --git a/include/exec/memory.h b/include/exec/memory.h
index 3dcadcf3a2..7458082455 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -2990,6 +2990,8 @@ static inline bool memory_access_is_direct(MemoryRegion 
*mr, bool is_write,
                                            MemTxAttrs attrs)
 {
     if (is_write) {
+        if (attrs.debug)
+            return memory_region_is_ram(mr) || memory_region_is_romd(mr);
         return memory_region_is_ram(mr) && !mr->readonly &&
                !mr->rom_device && !memory_region_is_ram_device(mr);
     } else {
diff --git a/system/physmem.c b/system/physmem.c
index 3c2a0b0289..03d3618039 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -3573,13 +3573,8 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
         if (l > len)
             l = len;
         phys_addr += (addr & ~TARGET_PAGE_MASK);
-        if (is_write) {
-            res = address_space_write_rom(cpu->cpu_ases[asidx].as, phys_addr,
-                                          attrs, buf, l);
-        } else {
-            res = address_space_read(cpu->cpu_ases[asidx].as, phys_addr,
-                                     attrs, buf, l);
-        }
+        res = address_space_rw(cpu->cpu_ases[asidx].as, phys_addr, attrs, buf,
+                               l, is_write);
         if (res != MEMTX_OK) {
             return -1;
         }
--
2.47.1


You could could give that a churn if that solves your issue as well.

Let me CC more people that might have an opinion on how it should be done.

--
Cheers,

David / dhildenb