Re: [RFC PATCH v1 3/6] x86/sev: Maintain shadow rmptable on Hyper-V

[Jeremi Piotrowski]

On Sun, Jan 29, 2023 at 04:37:24AM +0000, Michael Kelley (LINUX) wrote:
> From: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx> Sent: Monday, January 23, 2023 8:51 AM
> > 
> > Hyper-V can expose the SEV-SNP feature to guests, and manages the
> > system-wide RMP (Reverse Map) table. The SNP implementation in the
> > kernel needs access to the rmptable for tracking pages and deciding
> > when/how to issue rmpupdate/psmash.  When running as a Hyper-V guest
> > with SNP support, an rmptable is allocated by the kernel during boot for
> > this purpose. Keep the table in sync with issued rmpupdate/psmash
> > instructions.
> > 
> > The logic for how to update the rmptable comes from "AMD64 Architecture
> > Programmer’s Manual, Volume 3" which describes the psmash and rmpupdate
> > instructions. To ensure correctness of the SNP host code, the most
> > important fields are "assigned" and "page size".
> > 
> > Signed-off-by: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/sev.c | 59 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 59 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> > index 95404c7e5150..edec1ccb80b1 100644
> > --- a/arch/x86/kernel/sev.c
> > +++ b/arch/x86/kernel/sev.c
> > @@ -26,6 +26,7 @@
> >  #include <linux/iommu.h>
> >  #include <linux/amd-iommu.h>
> > 
> > +#include <asm/mshyperv.h>
> 
> The code in sev.c should be generic and appropriate for use with any
> hypervisor.  As such, I think we want to avoid sev.c having a dependency
> on Hyper-V specific code, such as the <asm/mshyperv.h> include file
> and the ms_hyperv.nested_features variable as used below.
> 
> Instead, create a boolean static variable in the sev.c module along with
> a wrapper function to set it.  The logic that tests hv_no_rmp_table()
> should test this static variable instead, which would default to "false".
> Hypervisor-specific initialization code can call the wrapper function
> to set the variable to "true" based on whatever logic is appropriate for
> the hypervisor.  This approach reverses the dependency and hopefully
> is usable by other hypervisors that want to offer nested SNP support.
> 

ok, this makes sense. I've added a call to the wrapper to the init_mem_mapping
callback added before so that it is enabled together with allocating the
rmptable.