Re: [PATCH v3] hw/i386/pc: improve physical address space bound check for 32-bit systems

[David Hildenbrand]

On 21.09.23 09:17, Ani Sinha wrote:
> 32-bit systems do not have a reserved memory for hole64 and hotplugging memory
> devices are not supported on those systems. Therefore, the maximum limit of the
> guest physical address in the absence of additional memory devices effectively
> coincides with the end of "above 4G memory space" region. When users configure
> additional memory devices, we need to properly account for the additional device
> memory region so as to find the maximum value of the guest physical address
> and enforce that it is within the physical address space of the processor. For
> 32-bit, this maximum PA will be outside the range of the processor's address
> space.
>
> With this change, for example, previously this was allowed:
>
> $ ./qemu-system-x86_64 -cpu pentium -m size=10G
>
> Now it is no longer allowed:
>
> $ ./qemu-system-x86_64 -cpu pentium -m size=10G
> qemu-system-x86_64: Address space limit 0xffffffff < 0x2bfffffff phys-bits too 
> low (32)
>
> For 32-bit, hotplugging additional memory is no longer allowed.

"32-bit without PAE/PSE36"

> $ ./qemu-system-i386 -m size=1G,maxmem=3G,slots=2
> qemu-system-i386: Address space limit 0xffffffff < 0x1ffffffff phys-bits too 
> low (32)

We always place the device memory region above 4G. Without PAE/PSE36, 
you cannot ever possibly make use hotplugged memory, because it would 
reside > 4g.

So while the user could have started that QEMU instance, even with an OS 
that would support memory hotplug, a DIMM above 4G would not have been 
usable.

So we're now properly failing for a setup that doesn't make any sense. 
Good :)

... if someone ever cares about making that work, we would have to let 
the device memory region start below 4g (and obviously, not exceed 4g).


So while

./qemu-system-i386 -m size=1G,maxmem=3G,slots=2

fails (because pentium cannot access that memory), what should work is

./qemu-system-i386 -m size=1G,maxmem=3G,slots=2 -cpu pentium2

or

 ./qemu-system-i386 -m size=1G,maxmem=3G,slots=2 -cpu pentium,pse36=on

Because that CPU could actually address that memory somehow (PAE/PSE36).


So IMHO, we're now forbidding setups that are impossible.

> The above is still allowed for older machine types in order to support
> compatibility. Therefore, this still works:
>
> $ ./qemu-system-i386 -machine pc-i440fx-8.1 -m size=1G,maxmem=3G,slots=2

Makes sense. (probably nobody cares, but better safe than sorry)

> After calling CPUID with EAX=0x80000001, all AMD64 compliant processors
> have the longmode-capable-bit turned on in the extended feature flags (bit 29)
> in EDX. The absence of CPUID longmode can be used to differentiate between
> 32-bit and 64-bit processors and is the recommended approach. QEMU takes this
> approach elsewhere (for example, please see x86_cpu_realizefn()) and with
> this change, pc_max_used_gpa() also takes the same approach to detect 32-bit
> processors.
>
> Unit tests are modified to not run those tests that use memory hotplug
> on 32-bit x86 architecture.

We could use a different CPU (pentium2) to still run these tests. 
"pentium2" should work I assume?
[...]

> @@ -907,12 +907,39 @@ static uint64_t pc_get_cxl_range_end(PCMachineState *pcms)
>   static hwaddr pc_max_used_gpa(PCMachineState *pcms, uint64_t pci_hole64_size)
>   {
>       X86CPU *cpu = X86_CPU(first_cpu);
> +    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
> +    MachineState *ms = MACHINE(pcms);
> +    uint64_t devmem_start = 0;
> +    ram_addr_t devmem_size = 0;
>   
> -    /* 32-bit systems don't have hole64 thus return max CPU address */
> -    if (cpu->phys_bits <= 32) {
> -        return ((hwaddr)1 << cpu->phys_bits) - 1;
> +    /*
> +     * 32-bit systems don't have hole64 but they might have a region for
> +     * memory devices. Even if additional hotplugged memory devices might
> +     * not be usable by most guest OSes, we need to still consider them for
> +     * calculating the highest possible GPA so that we can properly report
> +     * if someone configures them on a CPU that cannot possibly address them.
> +     */
> +    if (!(cpu->env.features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM)) {
> +        /* 32-bit systems */
> +        if (pcmc->fixed_32bit_mem_addr_check) {
> +            if (pcmc->has_reserved_memory &&
> +                (ms->ram_size < ms->maxram_size)) {
> +                pc_get_device_memory_range(pcms, &devmem_start,
> +                                           &devmem_size);
> +                if (!pcmc->broken_reserved_end) {

I think you can remove that check. "pcmc->fixed_32bit_mem_addr_check && 
pcmc->broken_reserved_end" can never hold at the same time.

broken_reserved_end is only set for QEMU <= 2.4, to work around another 
broken check. pcmc->fixed_32bit_mem_addr_check is only set for 8.2+.

Maybe consider calling "fixed_32bit_mem_addr_check" 
"pcmc->broken_32bit_max_gpa_check" and reverse the logic (treating it 
like broken_reserved_end).


--
Cheers,

David / dhildenb