Re: [PATCH v2 2/5] x86/kexec: do unconditional WBINVD in relocate_kernel()

[Tom Lendacky]

On 3/19/24 16:20, Huang, Kai wrote:
> On 20/03/2024 3:38 am, Tom Lendacky wrote:
> > On 3/19/24 06:13, Kirill A. Shutemov wrote:
> > > On Tue, Mar 19, 2024 at 01:48:45AM +0000, Kai Huang wrote:
> > > > Both SME and TDX can leave caches in incoherent state due to memory
> > > > encryption.  During kexec, the caches must be flushed before jumping to
> > > > the second kernel to avoid silent memory corruption to the second kernel.
> > > >
> > > > During kexec, the WBINVD in stop_this_cpu() flushes caches for all
> > > > remote cpus when they are being stopped.  For SME, the WBINVD in
> > > > relocate_kernel() flushes the cache for the last running cpu (which is
> > > > executing the kexec).
> > > >
> > > > Similarly, for TDX after stopping all remote cpus with cache flushed, to
> > > > support kexec, the kernel needs to flush cache for the last running cpu.
> > > >
> > > > Make the WBINVD in the relocate_kernel() unconditional to cover both SME
> > > > and TDX.
> > >
> > > Nope. It breaks TDX guest. WBINVD triggers #VE for TDX guests.
> >
> > Ditto for SEV-ES/SEV-SNP, a #VC is generated and crashes the guest.
>
> Oh I forgot these.
>
> Hi Kirill,
>
> Then I think patch 1 will also break TDX guest after your series to enable 
> multiple cpus for the second kernel after kexec()?
>
> Hi Tom,
>
> I am not aware of kexec() support status for SEV-ES/SEV-SNP guests. Does 
> patch 1 break them?

SNP guests can kexec with some patches that are currently in process 
around shared to private memory conversions. ES guests can only kexec with 
a single vCPU. There was a recent patch series to add support for multiple 
vCPUs.

Patch #1 doesn't break either ES or SNP because we still have an IDT and 
traditional kernel addressing in place, so the #VC can be handled.

Whereas patch #2 has switched to identity mapping and removed the IDT, so 
a #VC causes a triple fault.

Thanks,
Tom