lkml.org 
[lkml]   [2024]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v4 1/5] x86/kexec: do unconditional WBINVD for bare-metal in stop_this_cpu()
From


On 1/06/2024 8:45 am, Tom Lendacky wrote:
> On 5/22/24 21:49, Huang, Kai wrote:
>> On 18/04/2024 11:48 pm, Kai Huang wrote:
>>> TL;DR:
>>>
>>> Change to do unconditional WBINVD in stop_this_cpu() for bare metal
>>> to cover kexec support for both AMD SME and Intel TDX, despite there
>>> _was_ some issue preventing from doing so but now has it got fixed.
>>>
>>> Long version:
>>>
>>> Both AMD SME and Intel TDX can leave caches in an incoherent state due
>>> to memory encryption, which can lead to silent memory corruption during
>>> kexec.  To address this issue, it is necessary to flush the caches
>>> before jumping to the second kernel.
>>>
>>> Currently, the kernel only performs WBINVD in stop_this_cpu() when SME
>>> is supported by hardware.  To support TDX, instead of adding one more
>>> vendor-specific check, it is proposed to perform unconditional WBINVD.
>>> Kexec() is a slow path, and the additional WBINVD is acceptable for the
>>> sake of simplicity and maintainability.
>>>
>>
>> Hi Tom,
>>
>> May I ask how does SME work with kdump in crash_kexec().  Looking at
>> the code, AFAICT the crash_kexec() path doesn't use stop_this_cpu() to
>> stop all other cpus.  Instead, kdump_nmi_shootdown_cpus() is called to
>> send NMI to remote cpus and crash_nmi_callback() is invoked to stop them.
>>
>> But the crash_nmi_callback() doesn't invoke WBINVD for SME AFAICT.  It
>> does call the kdump_nmi_callback() callback where a WBINVD is
>> performed for the SNP host:
>>
>> void kdump_sev_callback(void)
>> {
>>          /*
>>           * Do wbinvd() on remote CPUs when SNP is enabled in order to
>>           * safely do SNP_SHUTDOWN on the local CPU.
>>           */
>>          if (cc_platform_has(CC_ATTR_HOST_SEV_SNP))
>>                  wbinvd();
>> }
>>
>> So if I read correctly, what's the reason the WBINVD is skipped for
>> SME in case of crash_kexec()?
>
> The system is rebooted after a crash and doesn't continue directly on
> into a new kernel.
>

How about the kdump kernel itself? Would the stale cachelines
potentially corrupt it?

And how about /proc/vmcore, which reflects the system RAM used by the
first, crashed, kernel? Is it OK to have stale cachelines for it?

\
 
 \ /
  Last update: 2024-06-04 02:58    [W:0.076 / U:1.220 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site