Re: [PATCH v4 1/5] x86/kexec: do unconditional WBINVD for bare-metal in stop_this_cpu()

[Tom Lendacky]

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.

Thanks,
Tom