Re: [PATCH v8 10/11] i386/pc: relocate 4g start to 1T where applicable

[Igor Mammedov]

On Fri, 15 Jul 2022 18:16:27 +0100
Joao Martins <joao.m.martins@oracle.com> wrote:

> It is assumed that the whole GPA space is available to be DMA
> addressable, within a given address space limit, except 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 boundary 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 or the address space covers it, we
> also add the the reserved HT e820 range as reserved.
> 
> Default phys-bits on Qemu is TCG_PHYS_ADDR_BITS (40) which is enough
> to address 1Tb (0xff ffff ffff). On AMD platforms, if a
> ram-above-4g relocation is attempted and the CPU wasn't configured
> with a big enough phys-bits, an error message will be printed
> due to the maxphysaddr vs maxusedaddr check previously added.
> 
> Suggested-by: Igor Mammedov <imammedo@redhat.com>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>

Acked-by: Igor Mammedov <imammedo@redhat.com>

> ---
>  hw/i386/pc.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 54 insertions(+)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index f30661b7f1a2..a71135930833 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -892,6 +892,40 @@ static hwaddr pc_max_used_gpa(PCMachineState *pcms, 
> uint64_t pci_hole64_size)
>      return pc_pci_hole64_start() + pci_hole64_size - 1;
>  }
>  
> +/*
> + * 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)
> +
>  void pc_memory_init(PCMachineState *pcms,
>                      MemoryRegion *system_memory,
>                      MemoryRegion *rom_memory,
> @@ -915,6 +949,26 @@ 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)) {
> +        /* Bail out if max possible address does not cross HT range */
> +        if (pc_max_used_gpa(pcms, pci_hole64_size) >= AMD_HT_START) {
> +            x86ms->above_4g_mem_start = AMD_ABOVE_1TB_START;
> +        }
> +
> +        /*
> +         * Advertise the HT region if address space covers the reserved
> +         * region or if we relocate.
> +         */
> +        if (cpu->phys_bits >= 40) {
> +            e820_add_entry(AMD_HT_START, AMD_HT_SIZE, E820_RESERVED);
> +        }
> +    }
> +
>      /*
>       * phys-bits is required to be appropriately configured
>       * to make sure max used GPA is reachable.