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

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Message-ID: <c7cec9118a23220986c1894f18cafb3aa5b9fc1f.camel@intel.com>
Date: Mon, 17 Mar 2025 21:59:50 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
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Subject: Re: [RFC PATCH 1/5] x86/kexec: Do unconditional WBINVD for bare-metal
 in stop_this_cpu()

On Mon, 2025-03-17 at 14:52 +0200, kirill.shutemov@...ux.intel.com wrote:
> On Thu, Mar 13, 2025 at 06:40:09PM +0000, Edgecombe, Rick P wrote:
> > > Currently, the kernel only performs WBINVD in stop_this_cpu() when SME
> > > is supported by hardware.  Perform unconditional WBINVD to support TDX
> > > instead of adding one more vendor-specific check.  Kexec is a slow path,
> > > and the additional WBINVD is acceptable for the sake of simplicity and
> > > maintainability.
> > 
> > Out of curiosity, do you know why this was not already needed for non-self snoop
> > CPUs? Why can't there be other cache modes that get written back after the new
> > kernel starts using the memory for something else?
> 
> KeyID is a hack. Memory controller is aware about KeyID, but not cache.
> Cache considers KeyID as part of physical address. Two cache lines for the
> same physical address with different KeyID are considered unrelated from
> cache coherency PoV.

Sure, but non-selfsnoop CPUs can have trouble when PAT aliases cachetypes, I
guess. This came up in KVM recently.

So if new kernel maps the same memory with a different memtype I thought it
might be a similar problem.

There is a little bit here, but it doesn't mention snooping. Not an expert in
this area, btw.
https://www.kernel.org/doc/Documentation/x86/pat.txt