Re: [PATCH] hw/i386/pc.c: CXL Fixed Memory Window should not reserve e820 in bios

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Wed, 26 Oct 2022 16:59:13 -0400
Gregory Price <gourry.memverge@gmail.com> wrote:

> Early-boot e820 records will be inserted by the bios/efi/early boot
> software and be reported to the kernel via insert_resource.  Later, when
> CXL drivers iterate through the regions again, they will insert another
> resource and make the RESERVED memory area a child.
> 
> This RESERVED memory area causes the memory region to become unusable,
> and as a result attempting to create memory regions with
> 
>     `cxl create-region ...`
> 
> Will fail due to the RESERVED area intersecting with the CXL window.
> 
> During boot the following traceback is observed:
> 
> 0xffffffff81101650 in insert_resource_expand_to_fit ()
> 0xffffffff83d964c5 in e820__reserve_resources_late ()
> 0xffffffff83e03210 in pcibios_resource_survey ()
> 0xffffffff83e04f4a in pcibios_init ()
> 
> Which produces a call to reserve the CFMWS area:
> 
> (gdb) p *new
> $54 = {start = 0x290000000, end = 0x2cfffffff, name = "Reserved",
>        flags = 0x200, desc = 0x7, parent = 0x0, sibling = 0x0,
>        child = 0x0}
> 
> Later the Kernel parses ACPI tables and reserves the exact same area as
> the CXL Fixed Memory Window:
> 
> 0xffffffff811016a4 in insert_resource_conflict ()
>                       insert_resource ()
> 0xffffffff81a81389 in cxl_parse_cfmws ()
> 0xffffffff818c4a81 in call_handler ()
>                       acpi_parse_entries_array ()
> 
> (gdb) p/x *new
> $59 = {start = 0x290000000, end = 0x2cfffffff, name = "CXL Window 0",
>        flags = 0x200, desc = 0x0, parent = 0x0, sibling = 0x0,
>        child = 0x0}
> 
> This produces the following output in /proc/iomem:
> 
> 590000000-68fffffff : CXL Window 0
>   590000000-68fffffff : Reserved
> 
> This reserved area causes `get_free_mem_region()` to fail due to a check
> against `__region_intersects()`.  Due to this reserved area, the
> intersect check will only ever return REGION_INTERSECTS, which causes
> `cxl create-region` to always fail.
> 
> Signed-off-by: Gregory Price <gregory.price@memverge.com>

My understanding of e820 is limited, but from discussions with Intel folk
I believe this fix to be correct - we should never have had e820 regions
for CXL Fixed Memory Windows. As such

Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> ---
>  hw/i386/pc.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 768982ae9a..203c90fedb 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1062,7 +1062,6 @@ void pc_memory_init(PCMachineState *pcms,
>          hwaddr cxl_size = MiB;
>  
>          cxl_base = pc_get_cxl_range_start(pcms);
> -        e820_add_entry(cxl_base, cxl_size, E820_RESERVED);
>          memory_region_init(mr, OBJECT(machine), "cxl_host_reg", cxl_size);
>          memory_region_add_subregion(system_memory, cxl_base, mr);
>          cxl_resv_end = cxl_base + cxl_size;
> @@ -1078,7 +1077,6 @@ void pc_memory_init(PCMachineState *pcms,
>                  memory_region_init_io(&fw->mr, OBJECT(machine), &cfmws_ops, fw,
>                                        "cxl-fixed-memory-region", fw->size);
>                  memory_region_add_subregion(system_memory, fw->base, &fw->mr);
> -                e820_add_entry(fw->base, fw->size, E820_RESERVED);
>                  cxl_fmw_base += fw->size;
>                  cxl_resv_end = cxl_fmw_base;
>              }