linux-kernel - Re: [PATCH] x86/tdx: Fix crash on kexec with CONFIG

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Message-ID: <e13df309-457a-41fa-9406-22476f9f4e72@amd.com>
Date: Wed, 28 Aug 2024 14:43:23 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Tom Lendacky <thomas.lendacky@....com>,
 "Maciej W. Rozycki" <macro@...am.me.uk>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Kevin Loughlin <kevinloughlin@...gle.com>, linux-kernel@...r.kernel.org,
 Rick Edgecombe <rick.p.edgecombe@...el.com>,
 Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH] x86/tdx: Fix crash on kexec with CONFIG_EISA

Hello Kirill,

On 8/28/2024 1:21 AM, Kirill A. Shutemov wrote:
> On Tue, Aug 27, 2024 at 05:15:56PM -0500, Kalra, Ashish wrote:
>> 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.
> It is not about testing a specific configuration. The question is if it
> safe for memremap() to map all WB memory as encrypted by default.
>
> Looks like it is safe for TDX, but I am not sure about SME/SEV.

For SEV it may make sense, but for SME we don't want memremap() to map all WB memory as encrypted by default.

>
> Maybe we want a specific flag to make memremap() map WB memory as
> decrypted/shared. Make everything encrypted by default seems like a sane
> default.

What are MEMREMAP_ENC, MEMREMAP_DEC flags being used for currently ?

Thanks, Ashish