linux-kernel - Re: [PATCH v4 1/5] x86/kexec: do unconditional WBINVD for bare-metal in stop_this

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Date: Thu, 23 May 2024 14:49:41 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: <linux-kernel@...r.kernel.org>
CC: <x86@...nel.org>, <dave.hansen@...el.com>, <bp@...en8.de>,
	<kirill.shutemov@...ux.intel.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<hpa@...or.com>, <luto@...nel.org>, <peterz@...radead.org>,
	<rick.p.edgecombe@...el.com>, <thomas.lendacky@....com>,
	<ashish.kalra@....com>, <chao.gao@...el.com>, <bhe@...hat.com>,
	<nik.borisov@...e.com>, <pbonzini@...hat.com>, <seanjc@...gle.com>
Subject: Re: [PATCH v4 1/5] x86/kexec: do unconditional WBINVD for bare-metal
 in stop_this_cpu()



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()?