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

[Michael S. Tsirkin]

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.



> +
>      if (x86ms->above_4g_mem_size > 0) {
>          ram_above_4g = g_malloc(sizeof(*ram_above_4g));
>          memory_region_init_alias(ram_above_4g, NULL, "ram-above-4g",
> -- 
> 2.17.2