linux-kernel - Re: [PATCH v2 2/5] x86/kexec: do unconditional WBINVD in relocate

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Date: Wed, 20 Mar 2024 08:49:57 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: "Huang, Kai" <kai.huang@...el.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, dave.hansen@...el.com,
 bp@...en8.de, tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
 luto@...nel.org, peterz@...radead.org, rick.p.edgecombe@...el.com,
 ashish.kalra@....com, chao.gao@...el.com, bhe@...hat.com,
 nik.borisov@...e.com, pbonzini@...hat.com, seanjc@...gle.com
Subject: Re: [PATCH v2 2/5] x86/kexec: do unconditional WBINVD in
 relocate_kernel()

On 3/19/24 16:20, Huang, Kai wrote:
> 
> 
> On 20/03/2024 3:38 am, Tom Lendacky wrote:
>> On 3/19/24 06:13, Kirill A. Shutemov wrote:
>>> On Tue, Mar 19, 2024 at 01:48:45AM +0000, Kai Huang wrote:
>>>> Both SME and TDX can leave caches in incoherent state due to memory
>>>> encryption.  During kexec, the caches must be flushed before jumping to
>>>> the second kernel to avoid silent memory corruption to the second kernel.
>>>>
>>>> During kexec, the WBINVD in stop_this_cpu() flushes caches for all
>>>> remote cpus when they are being stopped.  For SME, the WBINVD in
>>>> relocate_kernel() flushes the cache for the last running cpu (which is
>>>> executing the kexec).
>>>>
>>>> Similarly, for TDX after stopping all remote cpus with cache flushed, to
>>>> support kexec, the kernel needs to flush cache for the last running cpu.
>>>>
>>>> Make the WBINVD in the relocate_kernel() unconditional to cover both SME
>>>> and TDX.
>>>
>>> Nope. It breaks TDX guest. WBINVD triggers #VE for TDX guests.
>>
>> Ditto for SEV-ES/SEV-SNP, a #VC is generated and crashes the guest.
>>
> 
> Oh I forgot these.
> 
> Hi Kirill,
> 
> Then I think patch 1 will also break TDX guest after your series to enable 
> multiple cpus for the second kernel after kexec()?
> 
> Hi Tom,
> 
> I am not aware of kexec() support status for SEV-ES/SEV-SNP guests. Does 
> patch 1 break them?

SNP guests can kexec with some patches that are currently in process 
around shared to private memory conversions. ES guests can only kexec with 
a single vCPU. There was a recent patch series to add support for multiple 
vCPUs.

Patch #1 doesn't break either ES or SNP because we still have an IDT and 
traditional kernel addressing in place, so the #VC can be handled.

Whereas patch #2 has switched to identity mapping and removed the IDT, so 
a #VC causes a triple fault.

Thanks,
Tom