linux-kernel - Re: [PATCH v9 3/3] x86/snp: Convert shared memory back to private on kexec

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Date: Fri, 28 Jun 2024 14:14:14 -0500
From: "Kalra, Ashish" <ashish.kalra@....com>
To: Tom Lendacky <thomas.lendacky@....com>, Borislav Petkov <bp@...en8.de>
Cc: tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
 x86@...nel.org, rafael@...nel.org, hpa@...or.com, peterz@...radead.org,
 adrian.hunter@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
 jun.nakajima@...el.com, rick.p.edgecombe@...el.com, michael.roth@....com,
 seanjc@...gle.com, kai.huang@...el.com, bhe@...hat.com,
 kirill.shutemov@...ux.intel.com, bdas@...hat.com, vkuznets@...hat.com,
 dionnaglaze@...gle.com, anisinha@...hat.com, jroedel@...e.de,
 ardb@...nel.org, dyoung@...hat.com, kexec@...ts.infradead.org,
 linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 3/3] x86/snp: Convert shared memory back to private on
 kexec

Hello Tom,

On 6/28/2024 9:01 AM, Tom Lendacky wrote:
> On 6/27/24 23:27, Kalra, Ashish wrote:
>> Hello Boris,
>>
>> On 6/24/2024 10:59 PM, Borislav Petkov wrote:
>>> On Mon, Jun 24, 2024 at 03:57:34PM -0500, Kalra, Ashish wrote:
>>>> ...  Hence, added simple static functions make_pte_private() and
>>>> set_pte_enc() to make use of the more optimized snp_set_memory_private() to
>>>> use the GHCB instead of the MSR protocol. Additionally, make_pte_private()
>>>> adds check for GHCB addresses during unshare_all_memory() loop.
>>> IOW, what you're saying is: "Boris, what you're asking can't be done."
>>>
>>> Well, what *you're* asking - for me to maintain crap - can't be done either.
>>> So this will stay where it is.
>>>
>>> Unless you make a genuine effort and refactor the code...
>> There is an issue with calling __set_clr_pte_enc() here for the _bss_decrypted section being made private again,
>>
>> when calling __set_clr_pte_enc() on _bss_decrypted section pages, clflush_cache_range() will fail as __va()
>>
>> on this physical range fails as the bss_decrypted section pages are not in kernel direct map.
>>
>> Hence, clflush_cache_range() in __set_clr_pte_enc() causes an implicit page state change which is not resolved as below and causes fatal guest exit :
>>
>> qemu-system-x86_64: warning: memory fault: GPA 0x4000000 size 0x1000 flags 0x8 kvm_convert_memory start 0x4000000 size 0x1000 shared_to_private
>>
>> ...
>>
>> KVM: unknown exit reason 24 EAX=00000000 EBX=00000000 ECX=00000000 EDX=00000000 ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000 EIP=00000000 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
>>
>> ...
>>
>> This is the reason why i had to pass the vaddr to set_pte_enc(), added here for kexec code, so that i can use it for clflush_cache_range().
>>
>> So for specific cases such as this, where we can't call __set_clr_pte_enc() on _bss_decrypted section, we probably need a separate set_pte_enc().
> You can probably add a va parameter, that when not NULL, is used for the
> flush. If NULL, then use the __va() macro on the pa.
>
> Thanks,
> Tom

Yes, i have done exactly that.

Thanks, Ashish