Re: [PATCH] x86/tdx: Fix crash on kexec with CONFIG_EISA

["Kalra, Ashish" <ashish.kalra@amd.com>]

Hello Kirill,

On 8/26/2024 10:52 AM, Tom Lendacky wrote:
> On 8/26/24 07:25, Kirill A. Shutemov wrote:
>> On Sat, Aug 24, 2024 at 11:29:39PM +0100, Maciej W. Rozycki wrote:
>>> On Thu, 22 Aug 2024, Kirill A. Shutemov wrote:
>>>
>>>> This issue causes real problems:
>>>>
>>>> 1. If the kernel is compiled with EISA support, it will attempt to probe
>>>>    EISA by reading 4 bytes from the 0x0FFFD9 address (see eisa_bus_probe()).
>>>>    The kernel treats this read as MMIO and accesses this memory via
>>>>    shared mapping as we do for MMIO.
>>>>
>>>>    KVM converts memory to shared upon such access.
>>>>
>>>> 2. The same memory range (0xF0000-0x100000) is scanned to look for the MP
>>>>    table (see mpparse_find_mptable()). However, this is not MMIO and it
>>>>    is accessed via private mapping.
>>>>
>>>>    This will cause a crash if the memory is not private.
>>>>
>>>> During normal boot, the kernel scans for SMP information before probing
>>>> for EISA, and it boots fine. However, the memory becomes shared and causes
>>>> issues on kexec when the second kernel attempts to scan for SMP information.
>>>  ISTM that `eisa_bus_probe' has to be updated to `memremap' analogously to 
>>> `mpparse_find_mptable', complementing changes such as commit f7750a795687 
>>> ("x86, mpparse, x86/acpi, x86/PCI, x86/dmi, SFI: Use memremap() for RAM 
>>> mappings") or commit 5997efb96756 ("x86/boot: Use memremap() to map the 
>>> MPF and MPC data").  Both just access BIOS memory.
>>>
>>>  Can you please try and verify if my proposed change at: 
>>> <https://lore.kernel.org/r/alpine.DEB.2.21.2408242025210.30766@angie.orcam.me.uk> 
>>> has fixed the problem for you?
>> I like the direction your patch took. I hate sprinkling
>> X86_FEATURE_TDX_GUEST checks over the kernel.
>>
>> Unfortunately, it is not enough to fix the issue. memremap() in this case
>> will still boil down to ioremap() that would set shared bit:
>>
>> memremap()
>>   arch_memremap_wb()
>>     ioremap_cache()
>>       __ioremap_caller(.encrytped = false)
>>
>> I think arch_memremap_wb() should be mapped ioremap_encrypted() in x86
>> case. See the patch below.
>>
>> It seems to be working fine on TDX, but I am not sure about SEV.
>>
>> Tom, any comments?
> I haven't dug through the code that thoroughly, but I don't think making
> arch_memremap_wb() be ioremap_encrypted() will work for SME, where some
> data, e.g. setup data, is unencrypted and needs to be mapped shared.
>
> Let me add @Ashish to the thread and have him investigate this since he
> has been working on the kexec support under SNP. Can someone provide the
> specific kernel options that need to be in place?

As Tom asked for, please provide the specific kernel options to test with this configuration.

Thanks, Ashish

>
> Thanks,
> Tom
>
>> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
>> index 1d60427379c9..ef79cbef1ef8 100644
>> --- a/arch/x86/include/asm/io.h
>> +++ b/arch/x86/include/asm/io.h
>> @@ -180,6 +180,8 @@ extern void __iomem *ioremap_prot(resource_size_t offset, unsigned long size, un
>>  extern void __iomem *ioremap_encrypted(resource_size_t phys_addr, unsigned long size);
>>  #define ioremap_encrypted ioremap_encrypted
>>  
>> +#define arch_memremap_wb ioremap_encrypted
>> +
>>  /**
>>   * ioremap     -   map bus memory into CPU space
>>   * @offset:    bus address of the memory