linux-kernel - Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache flush during kexec

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Date: Tue, 20 Feb 2024 20:07:08 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "thomas.lendacky@....com" <thomas.lendacky@....com>, "bp@...en8.de"
	<bp@...en8.de>
CC: "Gao, Chao" <chao.gao@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>,
	"luto@...nel.org" <luto@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>, "hpa@...or.com"
	<hpa@...or.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"nik.borisov@...e.com" <nik.borisov@...e.com>, "bhe@...hat.com"
	<bhe@...hat.com>
Subject: Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache flush
 during kexec

On Tue, 2024-02-20 at 08:47 -0600, Tom Lendacky wrote:
> On 2/20/24 08:28, Borislav Petkov wrote:
> > On Mon, Feb 19, 2024 at 04:09:47PM -0600, Tom Lendacky wrote:
> > > That's why the '!(sev_status & MSR_AMD64_SEV_ENABLED)' works here.
> > 
> > I would've never figured that out just from staring at the test. :-\
> > 
> > > Basically, if you are bare-metal, it will return true. And it will only
> > > return true for machines that support SME and have the
> > > MSR_AMD64_SYSCFG_MEM_ENCRYPT bit set in SYS_CFG MSR because of where the
> > > 'cc_vendor = CC_VENDOR_AMD' assignment is. However, if you move the
> > > 'cc_vendor = CC_VENDOR_AMD' to before the if statement, then you will have
> > > the WBINVD called for any machine that supports SME, even if SME is not
> > > possible because the proper bit in the SYS_CFG MSR hasn't been set.
> > > 
> > > I know what I'm trying to say, let me know if it is making sense...
> > 
> > Yah, thanks for taking the time to explain.
> > 
> > Here's an even more radical idea:
> > 
> > Why not do WBINVD *unconditionally* on the CPU down path?
> > 
> > - it is the opposite of a fast path, i.e., no one cares
> > 
> > - it'll take care of every possible configuration without ugly logic
> > 
> > - it wouldn't hurt to have the caches nice and coherent before going
> >    down
> > 
> > Hmmm.
> 
> That's what I initially did, but errors were reported, see commit:
>    f23d74f6c66c ("x86/mm: Rework wbinvd, hlt operation in stop_this_cpu()")

This changelog only mentions "Some issues".  Do you know exactly what kind
issues did you see?  Are these issues only appeared on SME enabled system or
other non-SME-capable systems too?

Because ...

> 
> But that may be because of the issue solved by commit:
>    1f5e7eb7868e ("x86/smp: Make stop_other_cpus() more robust")

... the issue resolved in this commit seems to be hang.