Re: [PATCH v6 09/10] i386/pc: relocate 4g start to 1T where applicable

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

On 7/12/22 10:06, Igor Mammedov wrote:
> On Mon, 11 Jul 2022 21:03:28 +0100
> Joao Martins <joao.m.martins@oracle.com> wrote:
> 
>> On 7/11/22 16:31, Joao Martins wrote:
>>> On 7/11/22 15:52, Joao Martins wrote:  
>>>> On 7/11/22 13:56, Igor Mammedov wrote:  
>>>>> On Fri,  1 Jul 2022 17:10:13 +0100
>>>>> Joao Martins <joao.m.martins@oracle.com> wrote:
>>>>>  
>>>>>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>>>>>> index a79fa1b6beeb..07025b510540 100644
>>>>>> --- a/hw/i386/pc.c
>>>>>> +++ b/hw/i386/pc.c
>>>>>> @@ -907,6 +907,87 @@ static uint64_t pc_get_cxl_range_end(PCMachineState *pcms)
>>>>>>      return start;
>>>>>>  }
>>>>>>  
>>>>>> +static hwaddr pc_max_used_gpa(PCMachineState *pcms,
>>>>>> +                                hwaddr above_4g_mem_start,
>>>>>> +                                uint64_t pci_hole64_size)
>>>>>> +{
>>>>>> +    X86MachineState *x86ms = X86_MACHINE(pcms);
>>>>>> +  
>>>>>  
>>>>>> +    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;
>>>>>> +    }  
>>>>> this hunk still bothers me (nothing changed wrt v5 issues around it)
>>>>> issues recap: (
>>>>>  1. correctness of it
>>>>>  2. being limited to AMD only, while it seems pretty generic to me
>>>>>  3. should be a separate patch
>>>>> )
>>>>>  
>>>> How about I instead delete this hunk, and only call pc_set_amd_above_4g_mem_start()
>>>> when there's @above_4g_mem_size ? Like in pc_memory_init() I would instead:
>>>>
>>>> if (IS_AMD_CPU(&cpu->env) && x86ms->above_4g_mem_size) {
>>>>     hwaddr start = x86ms->above_4g_mem_start;
>>>>
>>>>     if (pc_max_used_gpa(pcms, start, pci_hole64_size) >= AMD_HT_START) {
>>>>         pc_set_amd_above_4g_mem_start(pcms, pci_hole64_size);
>>>>     }
>>>>     ...
>>>> }
>>>>
>>>> Given that otherwise it is impossible to ever encounter the 1T boundary.
>>>>  
>>>
>>> And while at it I would also remove any unneeded arguments from pc_max_used_gpa(),
>>> which would turn the function into this:
>>>
>>> +static hwaddr pc_max_used_gpa(uint64_t pci_hole64_size)
>>> +{
>>> +    return pc_pci_hole64_start() + pci_hole64_size;
>>> +}
>>>
>>> I would nuke the added helper if it wasn't for having 2 call sites in this patch.
>>>   
>>
>> Full patch diff further below -- after removing pc_max_used_gpa() and made further
>> cleanups given this code can be much simpler after using this approach.
>>
>>>> If not ... what other alternative would address your concern?
>>>>  
>>
>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>> index e178bbc4129c..1ded3faeffe0 100644
>> --- a/hw/i386/pc.c
>> +++ b/hw/i386/pc.c
>> @@ -882,6 +882,62 @@ static uint64_t pc_get_cxl_range_end(PCMachineState *pcms)
>>      return start;
>>  }
>>
>> +/*
>> + * AMD systems with an IOMMU have an additional hole close to the
>> + * 1Tb, which are special GPAs that cannot be DMA mapped. Depending
>> + * on kernel version, VFIO may or may not let you DMA map those ranges.
>> + * Starting Linux v5.4 we validate it, and can't create guests on AMD machines
>> + * with certain memory sizes. It's also wrong to use those IOVA ranges
>> + * in detriment of leading to IOMMU INVALID_DEVICE_REQUEST or worse.
>> + * The ranges reserved for Hyper-Transport are:
>> + *
>> + * FD_0000_0000h - FF_FFFF_FFFFh
>> + *
>> + * The ranges represent the following:
>> + *
>> + * Base Address   Top Address  Use
>> + *
>> + * FD_0000_0000h FD_F7FF_FFFFh Reserved interrupt address space
>> + * FD_F800_0000h FD_F8FF_FFFFh Interrupt/EOI IntCtl
>> + * FD_F900_0000h FD_F90F_FFFFh Legacy PIC IACK
>> + * FD_F910_0000h FD_F91F_FFFFh System Management
>> + * FD_F920_0000h FD_FAFF_FFFFh Reserved Page Tables
>> + * FD_FB00_0000h FD_FBFF_FFFFh Address Translation
>> + * FD_FC00_0000h FD_FDFF_FFFFh I/O Space
>> + * FD_FE00_0000h FD_FFFF_FFFFh Configuration
>> + * FE_0000_0000h FE_1FFF_FFFFh Extended Configuration/Device Messages
>> + * FE_2000_0000h FF_FFFF_FFFFh Reserved
>> + *
>> + * See AMD IOMMU spec, section 2.1.2 "IOMMU Logical Topology",
>> + * Table 3: Special Address Controls (GPA) for more information.
>> + */
>> +#define AMD_HT_START         0xfd00000000UL
>> +#define AMD_HT_END           0xffffffffffUL
>> +#define AMD_ABOVE_1TB_START  (AMD_HT_END + 1)
>> +#define AMD_HT_SIZE          (AMD_ABOVE_1TB_START - AMD_HT_START)
>> +
>> +static void pc_set_amd_above_4g_mem_start(PCMachineState *pcms,
>> +                                          hwaddr maxusedaddr)
>> +{
>> +    X86MachineState *x86ms = X86_MACHINE(pcms);
>> +    hwaddr maxphysaddr;
>> +
>> +    /*
>> +     * Relocating ram-above-4G requires more than TCG_PHYS_ADDR_BITS (40).
>> +     * So make sure phys-bits is required to be appropriately sized in order
>> +     * to proceed with the above-4g-region relocation and thus boot.
>> +     */
>> +    maxphysaddr = ((hwaddr)1 << X86_CPU(first_cpu)->phys_bits) - 1;
>> +    if (maxphysaddr < maxusedaddr) {
>> +        error_report("Address space limit 0x%"PRIx64" < 0x%"PRIx64
>> +                     " phys-bits too low (%u) cannot avoid AMD HT range",
>> +                     maxphysaddr, maxusedaddr, X86_CPU(first_cpu)->phys_bits);
>> +        exit(EXIT_FAILURE);
>> +    }
>> +
>> +    x86ms->above_4g_mem_start = AMD_ABOVE_1TB_START;
>> +}
>> +
>>  void pc_memory_init(PCMachineState *pcms,
>>                      MemoryRegion *system_memory,
>>                      MemoryRegion *rom_memory,
>> @@ -897,6 +953,7 @@ void pc_memory_init(PCMachineState *pcms,
>>      PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
>>      X86MachineState *x86ms = X86_MACHINE(pcms);
>>      hwaddr cxl_base, cxl_resv_end = 0;
>> +    X86CPU *cpu = X86_CPU(first_cpu);
>>
>>      assert(machine->ram_size == x86ms->below_4g_mem_size +
>>                                  x86ms->above_4g_mem_size);
>> @@ -904,6 +961,29 @@ void pc_memory_init(PCMachineState *pcms,
>>      linux_boot = (machine->kernel_filename != NULL);
>>
>>      /*
>> +     * The HyperTransport range close to the 1T boundary is unique to AMD
>> +     * hosts with IOMMUs enabled. Restrict the ram-above-4g relocation
>> +     * to above 1T to AMD vCPUs only.
>> +     */
>> +    if (IS_AMD_CPU(&cpu->env) && x86ms->above_4g_mem_size) {
> 
> it has the same issue as pc_max_used_gpa(), i.e.
>   x86ms->above_4g_mem_size != 0
> doesn't mean that there isn't any memory above 4Gb nor that there aren't
> any MMIO (sgx/cxl/pci64hole), that's was the reason we were are considering
> max_used_gpa
> 
Argh yes, you are right. I see it now.

> I'd prefer to keep pc_max_used_gpa(),
> idea but make it work for above cases and be more generic (i.e. not to be
> tied to AMD only) since 'pc_max_used_gpa() < physbits'

Are you also indirectly suggesting here that the check inside
pc_set_amd_above_4g_mem_start() should be moved into pc_memory_init()
given that it's orthogonal to this issue. ISTR that you suggested this
at some point. If so, then there's probably very little reason to keep
pc_set_amd_above_4g_mem_start() around.

> applies to equally
> to AMD and Intel (and to trip it, one just have to configure small enough
> physbits or large enough hotpluggable RAM/CXL/PCI64HOLE)
> 
I can reproduce the issue you're thinking with basic memory hotplug. Let me see
what I can come up in pc_max_used_gpa() to cover this one. I'll respond here with a proposal.

I would really love to have v7.1.0 with this issue fixed but I am not very
confident it is going to make it :(

Meanwhile, let me know if you have thoughts on this one:

https://lore.kernel.org/qemu-devel/1b2fa957-74f6-b5a9-3fc1-65c5d68300ce@oracle.com/

I am going to assume that if no comments on the above that I'll keep things as is.

And also, whether I can retain your ack with Bernhard's suggestion here:

https://lore.kernel.org/qemu-devel/0eefb382-4ac6-4335-ca61-035babb95a88@oracle.com/

>> +        hwaddr maxusedaddr = pc_pci_hole64_start() + pci_hole64_size;
>> +
>> +        /* Bail out if max possible address does not cross HT range */
>> +        if (maxusedaddr >= AMD_HT_START) {
>> +            pc_set_amd_above_4g_mem_start(pcms, maxusedaddr);
>> +        }
>> +
>> +        /*
>> +         * Advertise the HT region if address space covers the reserved
>> +         * region or if we relocate.
>> +         */
>> +        if (x86ms->above_4g_mem_start == AMD_ABOVE_1TB_START ||
>> +            cpu->phys_bits >= 40) {
>> +            e820_add_entry(AMD_HT_START, AMD_HT_SIZE, E820_RESERVED);
>> +        }
>> +    }
>> +
>> +    /*
>>       * Split single memory region and use aliases to address portions of it,
>>       * done for backwards compatibility with older qemus.
>>       */
>>
>