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

[Joao Martins]

On 2/14/22 13:15, David Edmondson wrote:
> On Monday, 2022-02-07 at 20:24:21 GMT, Joao Martins wrote:
> 
>> Default phys-bits on Qemu is TCG_PHYS_BITS (40) which is enough
>> to address 1Tb (0xff ffff ffff). On AMD platforms, if a
>> ram-above-4g relocation happens and the CPU wasn't configured
>> with a big enough phys-bits, warn the user. There isn't a
>> catastrophic failure exactly, the guest will still boot, but
>> most likely won't be able to use more than ~4G of RAM.
>>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> ---
>>  hw/i386/pc.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> 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)
> 
> Braces around the block are required, I believe.
> 
Hmmm, my distration sorry about that -- checkpatch.pl wasn't so keen on warn me.

>> +        warn_report("Relocated RAM above 4G to start at %lu "
> 
> Should use PRIu64?
> 
Yeap, will do.

>> +                    "phys-bits too low (%u)",
>> +                    AMD_ABOVE_1TB_START, X86_CPU(first_cpu)->phys_bits);
>> +
>>      x86ms->above_4g_mem_start = AMD_ABOVE_1TB_START;
> 
> And a real nit - until above_4g_mem_start is modified, the number of
> phys_bits is fine, so I would have put the warning after the assignment.
> 
Good point. I'll reverse the order.

Thanks!
        Joao