Re: [PATCH v5 4/5] i386/pc: relocate 4g start to 1T where applicable

[Joao Martins]

On 6/20/22 17:36, Joao Martins wrote:
> On 6/20/22 15:27, Igor Mammedov wrote:
>> On Fri, 17 Jun 2022 14:33:02 +0100
>> Joao Martins <joao.m.martins@oracle.com> wrote:
>>> On 6/17/22 13:32, Igor Mammedov wrote:
>>>> On Fri, 17 Jun 2022 13:18:38 +0100
>>>> Joao Martins <joao.m.martins@oracle.com> wrote:  
>>>>> On 6/16/22 15:23, Igor Mammedov wrote:  
>>>>>> On Fri, 20 May 2022 11:45:31 +0100
>>>>>> Joao Martins <joao.m.martins@oracle.com> wrote:  
>>>>>>> +                                hwaddr above_4g_mem_start,
>>>>>>> +                                uint64_t pci_hole64_size)
>>>>>>> +{
>>>>>>> +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
>>>>>>> +    X86MachineState *x86ms = X86_MACHINE(pcms);
>>>>>>> +    MachineState *machine = MACHINE(pcms);
>>>>>>> +    ram_addr_t device_mem_size = 0;
>>>>>>> +    hwaddr base;
>>>>>>> +
>>>>>>> +    if (!x86ms->above_4g_mem_size) {
>>>>>>> +       /*
>>>>>>> +        * 32-bit pci hole goes from
>>>>>>> +        * end-of-low-ram (@below_4g_mem_size) to IOAPIC.
>>>>>>> +        */
>>>>>>> +        return IO_APIC_DEFAULT_ADDRESS - 1;    
>>>>>>
>>>>>> lack of above_4g_mem, doesn't mean absence of device_mem_size or 
>>>>>> anything else
>>>>>> that's located above it.
>>>>>>     
>>>>>
>>>>> True. But the intent is to fix 32-bit boundaries as one of the qtests was 
>>>>> failing
>>>>> otherwise. We won't hit the 1T hole, hence a nop.  
>>>>
>>>> I don't get the reasoning, can you clarify it pls?
>>>>   
>>>
>>> I was trying to say that what lead me here was a couple of qtests failures 
>>> (from v3->v4).
>>>
>>> I was doing this before based on pci_hole64. phys-bits=32 was for example 
>>> one
>>> of the test failures, and pci-hole64 sits above what 32-bit can reference.
>>
>> if user sets phys-bits=32, then nothing above 4Gb should work (be usable)
>> (including above-4g-ram, hotplug region or pci64 hole or sgx or cxl)
>>
>> and this doesn't look to me as AMD specific issue
>>
>> perhaps do a phys-bits check as a separate patch
>> that will error out if max_used_gpa is above phys-bits limit
>> (maybe at machine_done time)
>> (i.e. defining max_gpa and checking if compatible with configured cpu
>> are 2 different things)
>>
>> (it might be possible that tests need to be fixed too to account for it)
>>
> 
> My old notes (from v3) tell me with such a check these tests were exiting 
> early thanks to
> that error:
> 
>  1/56 qemu:qtest+qtest-x86_64 / qtest-x86_64/qom-test               ERROR     
>       0.07s
>   killed by signal 6 SIGABRT
>  4/56 qemu:qtest+qtest-x86_64 / qtest-x86_64/test-hmp               ERROR     
>       0.07s
>   killed by signal 6 SIGABRT
>  7/56 qemu:qtest+qtest-x86_64 / qtest-x86_64/boot-serial-test       ERROR     
>       0.07s
>   killed by signal 6 SIGABRT
> 44/56 qemu:qtest+qtest-x86_64 / qtest-x86_64/test-x86-cpuid-compat  ERROR     
>       0.09s
>   killed by signal 6 SIGABRT
> 45/56 qemu:qtest+qtest-x86_64 / qtest-x86_64/numa-test              ERROR     
>       0.17s
>   killed by signal 6 SIGABRT
> 
> But the real reason these fail is not at all related to CPU phys bits,
> but because we just don't handle the case where no pci_hole64 is supposed to 
> exist (which
> is what that other check is trying to do) e.g. A VM with -m 1G would
> observe the same thing i.e. the computations after that conditional are all 
> for the pci
> hole64, which acounts for SGX/CXL/hotplug or etc which consequently means 
> it's *errousnly*
> bigger than phys-bits=32 (by definition). So the error_report is just telling 
> me that
> pc_max_used_gpa() is just incorrect without the !x86ms->above_4g_mem_size 
> check.
> 
> If you're not fond of:
> 
> +    if (!x86ms->above_4g_mem_size) {
> +       /*
> +        * 32-bit pci hole goes from
> +        * end-of-low-ram (@below_4g_mem_size) to IOAPIC.
> +         */
> +        return IO_APIC_DEFAULT_ADDRESS - 1;
> +    }
> 
> Then what should I use instead of the above?
> 
> 'IO_APIC_DEFAULT_ADDRESS - 1' is the size of the 32-bit PCI hole, which is
> also what is used for i440fx/q35 code. I could move it to a macro (e.g.
> PCI_HOST_HOLE32_SIZE) to make it a bit readable and less hardcoded. Or
> perhaps your problem is on !x86ms->above_4g_mem_size and maybe I should check
> in addition for hotplug/CXL/etc existence?
> 
>>>>>  Unless we plan on using
>>>>> pc_max_used_gpa() for something else other than this.  
>>>>
>>>> Even if '!above_4g_mem_sizem', we can still have hotpluggable memory region
>>>> present and that can  hit 1Tb. The same goes for pci64_hole if it's 
>>>> configured
>>>> large enough on CLI.
>>>>   
>>> So hotpluggable memory seems to assume it sits above 4g mem.
>>>
>>> pci_hole64 likewise as it uses similar computations as hotplug.
>>>
>>> Unless I am misunderstanding something here.
>>>
>>>> Looks like guesstimate we could use is taking pci64_hole_end as max used 
>>>> GPA
>>>>   
>>> I think this was what I had before (v3[0]) and did not work.
>>
>> that had been tied to host's phys-bits directly, all in one patch
>> and duplicating existing pc_pci_hole64_start().
>>  
> 
> Duplicating was sort of my bad attempt in this patch for pc_max_used_gpa()
> 
> I was sort of thinking to something like extracting calls to start + size 
> "tuple" into
> functions -- e.g. for hotplug it is pc_get_device_memory_range() and for CXL 
> it would be
> maybe pc_get_cxl_range()) -- rather than assuming those values are already 
> initialized on
> the memory-region @base and its size.
> 
> See snippet below. Note I am missing CXL handling, but gives you the idea.
> 
> But it is slightly more complex than what I had in this version :( and would 
> require
> anyone doing changes in pc_memory_init() and pc_pci_hole64_start() to make 
> sure it follows
> the similar logic.
> 

Ignore previous snippet, here's a slightly cleaner version:

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 8eaa32ee2106..1d97c77a5eac 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -803,6 +803,43 @@ void xen_load_linux(PCMachineState *pcms)
 #define PC_ROM_ALIGN       0x800
 #define PC_ROM_SIZE        (PC_ROM_MAX - PC_ROM_MIN_VGA)

+static void pc_get_device_memory_range(PCMachineState *pcms,
+                                       hwaddr *base,
+                                       hwaddr *device_mem_size)
+{
+    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
+    X86MachineState *x86ms = X86_MACHINE(pcms);
+    MachineState *machine = MACHINE(pcms);
+    hwaddr addr, size;
+
+    if (pcmc->has_reserved_memory &&
+        machine->device_memory && machine->device_memory->base) {
+        addr = machine->device_memory->base;
+        size = memory_region_size(&machine->device_memory->mr);
+        goto out;
+    }
+
+    /* uninitialized memory region */
+    size = machine->maxram_size - machine->ram_size;
+
+    if (pcms->sgx_epc.size != 0) {
+        addr = sgx_epc_above_4g_end(&pcms->sgx_epc);
+    } else {
+        addr = x86ms->above_4g_mem_start + x86ms->above_4g_mem_size;
+    }
+
+    if (pcmc->enforce_aligned_dimm) {
+        /* size device region assuming 1G page max alignment per slot */
+        size += (1 * GiB) * machine->ram_slots;
+    }
+
+out:
+    if (base)
+        *base = addr;
+    if (device_mem_size)
+        *device_mem_size = size;
+}
+
 void pc_memory_init(PCMachineState *pcms,
                     MemoryRegion *system_memory,
                     MemoryRegion *rom_memory,
@@ -864,7 +901,7 @@ void pc_memory_init(PCMachineState *pcms,
     /* initialize device memory address space */
     if (pcmc->has_reserved_memory &&
         (machine->ram_size < machine->maxram_size)) {
-        ram_addr_t device_mem_size = machine->maxram_size - machine->ram_size;
+        ram_addr_t device_mem_size;

         if (machine->ram_slots > ACPI_MAX_RAM_SLOTS) {
             error_report("unsupported amount of memory slots: %"PRIu64,
@@ -879,20 +916,7 @@ void pc_memory_init(PCMachineState *pcms,
             exit(EXIT_FAILURE);
         }

-        if (pcms->sgx_epc.size != 0) {
-            machine->device_memory->base = 
sgx_epc_above_4g_end(&pcms->sgx_epc);
-        } else {
-            machine->device_memory->base =
-                x86ms->above_4g_mem_start + x86ms->above_4g_mem_size;
-        }
-
-        machine->device_memory->base =
-            ROUND_UP(machine->device_memory->base, 1 * GiB);
-
-        if (pcmc->enforce_aligned_dimm) {
-            /* size device region assuming 1G page max alignment per slot */
-            device_mem_size += (1 * GiB) * machine->ram_slots;
-        }
+        pc_get_device_memory_range(pcms, &machine->device_memory->base, 
&device_mem_size);

         if ((machine->device_memory->base + device_mem_size) <
             device_mem_size) {
@@ -965,12 +989,13 @@ uint64_t pc_pci_hole64_start(void)
     PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
     MachineState *ms = MACHINE(pcms);
     X86MachineState *x86ms = X86_MACHINE(pcms);
-    uint64_t hole64_start = 0;
+    uint64_t hole64_start = 0, size = 0;

-    if (pcmc->has_reserved_memory && ms->device_memory->base) {
-        hole64_start = ms->device_memory->base;
+    if (pcmc->has_reserved_memory &&
+        (ms->ram_size < ms->maxram_size)) {
+        pc_get_device_memory_range(pcms, &hole64_start, &size);
         if (!pcmc->broken_reserved_end) {
-            hole64_start += memory_region_size(&ms->device_memory->mr);
+            hole64_start += size;
         }
     } else if (pcms->sgx_epc.size != 0) {
             hole64_start = sgx_epc_above_4g_end(&pcms->sgx_epc);