linux-kernel - Re: [PATCH v5 07/22] x86/virt/tdx: Implement SEAMCALL function

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Message-ID: <59a2748ed446f3e8a00834982b54848937a97379.camel@intel.com>
Date:   Thu, 28 Jul 2022 00:46:56 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v5 07/22] x86/virt/tdx: Implement SEAMCALL function

On Tue, 2022-07-26 at 17:50 -0700, Dave Hansen wrote:
> On 7/26/22 17:34, Kai Huang wrote:
> > > This doesn't seem right to me.  *If* we get a known-bogus
> > > hot-remove event, we need to reject it.  Remember, removal is a
> > > two-step process.
> > If so, we need to reject the (CMR) memory offline.  Or we just BUG()
> > in the ACPI memory removal  callback?
> > 
> > But either way this will requires us to get the CMRs during kernel boot.
> 
> I don't get the link there between CMRs at boot and handling hotplug.
> 
> We don't need to go to extreme measures just to get a message out of the
> kernel that the BIOS is bad.  If we don't have the data to do it
> already, then I don't really see the nee to warn about it.
> 
> Think of a system that has TDX enabled in the BIOS, but is running an
> old kernel.  It will have *ZERO* idea that hotplug doesn't work.  It'll
> run blissfully along.  I don't see any reason that a kernel with TDX
> support, but where TDX is disabled should actively go out and try to be
> better than those old pre-TDX kernels.

Agreed, assuming "where TDX is disabled" you mean TDX isn't usable (i.e. when
TDX module isn't loaded, or won't be initialized at all).

> 
> Further, there's nothing to stop non-CMR memory from being added to a
> system with TDX enabled in the BIOS but where the kernel is not using
> it.  If we actively go out and keep good old DRAM from being added, then
> we unnecessarily addle those systems.
> 

OK.

Then for memory hot-add, perhaps we can just go with the "winner-take-all"
approach you mentioned before?

For memory hot-removal, as I replied previously, looks the kernel cannot reject
the removal if it allows memory offline.  Any suggestion on this?

-- 
Thanks,
-Kai