Re: [PATCH v4 4/5] i386/pc: relocate 4g start to 1T where applicable

[Joao Martins]

On 5/13/22 13:33, Michael S. Tsirkin wrote:
> On Wed, Apr 20, 2022 at 09:11:37PM +0100, Joao Martins wrote:
>> It is assumed that the whole GPA space is available to be DMA
>> addressable, within a given address space limit, expect for a
>> tiny region before the 4G. Since Linux v5.4, VFIO validates
>> whether the selected GPA is indeed valid i.e. not reserved by
>> IOMMU on behalf of some specific devices or platform-defined
>> restrictions, and thus failing the ioctl(VFIO_DMA_MAP) with
>>  -EINVAL.
>>
>> AMD systems with an IOMMU are examples of such platforms and
>> particularly may only have these ranges as allowed:
>>
>>      0000000000000000 - 00000000fedfffff (0      .. 3.982G)
>>      00000000fef00000 - 000000fcffffffff (3.983G .. 1011.9G)
>>      0000010000000000 - ffffffffffffffff (1Tb    .. 16Pb[*])
>>
>> We already account for the 4G hole, albeit if the guest is big
>> enough we will fail to allocate a guest with  >1010G due to the
>> ~12G hole at the 1Tb boundary, reserved for HyperTransport (HT).
>>
>> [*] there is another reserved region unrelated to HT that exists
>> in the 256T boundaru in Fam 17h according to Errata #1286,
>> documeted also in "Open-Source Register Reference for AMD Family
>> 17h Processors (PUB)"
>>
>> When creating the region above 4G, take into account that on AMD
>> platforms the HyperTransport range is reserved and hence it
>> cannot be used either as GPAs. On those cases rather than
>> establishing the start of ram-above-4g to be 4G, relocate instead
>> to 1Tb. See AMD IOMMU spec, section 2.1.2 "IOMMU Logical
>> Topology", for more information on the underlying restriction of
>> IOVAs.
>>
>> After accounting for the 1Tb hole on AMD hosts, mtree should
>> look like:
>>
>> 0000000000000000-000000007fffffff (prio 0, i/o):
>>       alias ram-below-4g @pc.ram 0000000000000000-000000007fffffff
>> 0000010000000000-000001ff7fffffff (prio 0, i/o):
>>      alias ram-above-4g @pc.ram 0000000080000000-000000ffffffffff
>>
>> If the relocation is done, we also add the the reserved HT
>> e820 range as reserved.
>>
>> 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 may be desired and the CPU wasn't configured
>> with a big enough phys-bits, print an error message to the user
>> and do not make the relocation of the above-4g-region if phys-bits
>> is too low.
>>
>> Suggested-by: Igor Mammedov <imammedo@redhat.com>
>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>> ---
>>  hw/i386/pc.c | 111 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 111 insertions(+)
>>
>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>> index 8eaa32ee2106..aac32ba0bd02 100644
>> --- a/hw/i386/pc.c
>> +++ b/hw/i386/pc.c
>> @@ -803,6 +803,110 @@ void xen_load_linux(PCMachineState *pcms)
>>  #define PC_ROM_ALIGN       0x800
>>  #define PC_ROM_SIZE        (PC_ROM_MAX - PC_ROM_MIN_VGA)
>>  
>> +/*
>> + * 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 hwaddr x86_max_phys_addr(PCMachineState *pcms,
>> +                                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;
>> +    }
>> +
>> +    if (pcmc->has_reserved_memory &&
>> +       (machine->ram_size < machine->maxram_size)) {
>> +        device_mem_size = machine->maxram_size - machine->ram_size;
>> +    }
>> +
>> +    base = ROUND_UP(above_4g_mem_start + x86ms->above_4g_mem_size +
>> +                    pcms->sgx_epc.size, 1 * GiB);
>> +
>> +    return base + device_mem_size + pci_hole64_size;
>> +}
>> +
>> +static void x86_update_above_4g_mem_start(PCMachineState *pcms,
>> +                                          uint64_t pci_hole64_size)
>> +{
>> +    X86MachineState *x86ms = X86_MACHINE(pcms);
>> +    CPUX86State *env = &X86_CPU(first_cpu)->env;
>> +    hwaddr start = x86ms->above_4g_mem_start;
>> +    hwaddr maxphysaddr, maxusedaddr;
>> +
>> +    /*
>> +     * 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(env)) {
>> +        return;
>> +    }
>> +
>> +    /* Bail out if max possible address does not cross HT range */
>> +    if (x86_max_phys_addr(pcms, start, pci_hole64_size) < AMD_HT_START) {
>> +        return;
>> +    }
>> +
>> +    /*
>> +     * Relocating ram-above-4G requires more than TCG_PHYS_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.
>> +     */
>> +    start = AMD_ABOVE_1TB_START;
>> +    maxphysaddr = ((hwaddr)1 << X86_CPU(first_cpu)->phys_bits) - 1;
>> +    maxusedaddr = x86_max_phys_addr(pcms, start, pci_hole64_size);
>> +    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 = start;
>> +}
>> +
>>  void pc_memory_init(PCMachineState *pcms,
>>                      MemoryRegion *system_memory,
>>                      MemoryRegion *rom_memory,
>> @@ -823,6 +927,8 @@ void pc_memory_init(PCMachineState *pcms,
>>  
>>      linux_boot = (machine->kernel_filename != NULL);
>>  
>> +    x86_update_above_4g_mem_start(pcms, pci_hole64_size);
>> +
>>      /*
>>       * Split single memory region and use aliases to address portions of it,
>>       * done for backwards compatibility with older qemus.
>> @@ -833,6 +939,11 @@ void pc_memory_init(PCMachineState *pcms,
>>                               0, x86ms->below_4g_mem_size);
>>      memory_region_add_subregion(system_memory, 0, ram_below_4g);
>>      e820_add_entry(0, x86ms->below_4g_mem_size, E820_RAM);
>> +
>> +    if (x86ms->above_4g_mem_start == AMD_ABOVE_1TB_START) {
>> +        e820_add_entry(AMD_HT_START, AMD_HT_SIZE, E820_RESERVED);
>> +    }
> 
> 
> Causes a warning (and so a build failure) on 32 bit mingw:
> 
> ../qemu/hw/i386/pc.c: In function 'pc_memory_init':
> ../qemu/hw/i386/pc.c:939:35: error: comparison is always false due to limited 
> range of data type [-Werror=type-limits]
>   939 |     if (x86ms->above_4g_mem_start == AMD_ABOVE_1TB_START) {
>       |                                   ^~
> cc1: all warnings being treated as errors
> 
> 
> Looking at the code, how is it supposed to work on 32 bit?
> It's ok if it does not work but I'd like a graceful failure
> not a silent corruption.
> 
> 
It didn't work on 32-bit qemu binaries -- My apologies for my oversight.

This diff below fixes the 2 warnings you got.
The rest of the added code uses hwaddr which is defined
as a uint64_t already (...)

diff --git a/include/hw/i386/x86.h b/include/hw/i386/x86.h
index 845e67c13f5a..b4022888879e 100644
--- a/include/hw/i386/x86.h
+++ b/include/hw/i386/x86.h
@@ -57,7 +57,7 @@ struct X86MachineState {
     ram_addr_t below_4g_mem_size, above_4g_mem_size;

     /* Start address of the initial RAM above 4G */
-    ram_addr_t above_4g_mem_start;
+    uint64_t above_4g_mem_start;

     /* CPU and apic information: */
     bool apic_xrupt_override;

(...) The second warning aforementioned being this:

../hw/i386/x86.c: In function 'x86_machine_initfn':
../hw/i386/x86.c:1324:33: error: large integer implicitly truncated to unsigned 
type
[-Werror=overflow]
     x86ms->above_4g_mem_start = 0x100000000ULL;
                                 ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors