Re: [PATCH RFCv2 3/4] i386/pc: warn if phys-bits is too low

[Joao Martins <joao.m.martins@oracle.com>]

On 2/14/22 15:41, Igor Mammedov wrote:
> On Mon, 14 Feb 2022 15:18:43 +0000
> Joao Martins <joao.m.martins@oracle.com> wrote:
>> On 2/14/22 15:03, Igor Mammedov wrote:
>>> On Mon,  7 Feb 2022 20:24:21 +0000
>>> Joao Martins <joao.m.martins@oracle.com> wrote:
>>>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>>>> index b060aedd38f3..f8712eb8427e 100644
>>>> --- a/hw/i386/pc.c
>>>> +++ b/hw/i386/pc.c
>>>> @@ -842,6 +842,7 @@ static void relocate_4g(MachineState *machine, PCMachineState *pcms)
>>>>      X86MachineState *x86ms = X86_MACHINE(pcms);
>>>>      ram_addr_t device_mem_size = 0;
>>>>      uint32_t eax, vendor[3];
>>>> +    hwaddr maxphysaddr;
>>>>  
>>>>      host_cpuid(0x0, 0, &eax, &vendor[0], &vendor[2], &vendor[1]);
>>>>      if (!IS_AMD_VENDOR(vendor)) {
>>>> @@ -858,6 +859,12 @@ static void relocate_4g(MachineState *machine, PCMachineState *pcms)
>>>>          return;
>>>>      }
>>>>  
>>>> +    maxphysaddr = ((hwaddr)1 << X86_CPU(first_cpu)->phys_bits) - 1;
>>>> +    if (maxphysaddr < AMD_ABOVE_1TB_START)
>>>> +        warn_report("Relocated RAM above 4G to start at %lu "
>>>> +                    "phys-bits too low (%u)",
>>>> +                    AMD_ABOVE_1TB_START, X86_CPU(first_cpu)->phys_bits);  
>>>
>>> perhaps this hunk belongs to the end of pc_memory_init(),
>>> it's not HT fixup specific at all?
>>>   
>> It is HT fixup related. Because we are relocating the whole above-4g-ram,
>> on what used to be enough with just 40 phys bits (default).
> 
> what if user starts QEMU with amount of RAM that won't fit into
> configured phys bits (whether it's default one or one that comes from host)
> and not on AMD host at that so no relocation happens but we still have
> broken configuration. What matters here is the end address that might
> be used by guest (pci64_bit hole end) is reachable.
> 
> That's why I suggested to make it generic check instead of AMD specific one. 
>  
OK, I see.

If I'm being dense, that would require replacing AMD_ABOVE_1TB_START in the
comparison to something that computes the max used addr -- Let me try that then.