Re: [PATCH v5 5/5] x86/kvm: Avoid dynamic allocation of pvclock data when SEV is active

[Brijesh Singh]

On 09/06/2018 01:47 PM, Sean Christopherson wrote:
> On Thu, Sep 06, 2018 at 01:37:50PM -0500, Brijesh Singh wrote:
> > On 09/06/2018 09:18 AM, Sean Christopherson wrote:
> > ....
> >
> > > > > So are we going to be defining a decrypted section for every piece of
> > > > > machinery now?
> > > > >
> > > > > That's a bit too much in my book.
> > > > >
> > > > > Why can't you simply free everything in .data..decrypted on !SVE guests?
> > > >
> > > > That would prevent adding __decrypted to existing declarations, e.g.
> > > > hv_clock_boot, which would be ugly in its own right.  A more generic
> > > > solution would be to add something like __decrypted_exclusive to mark
> > > > data that is used if and only if SEV is active, and then free the
> > > > SEV-only data when SEV is disabled.
> > >
> > > Oh, and we'd need to make sure __decrypted_exclusive is freed when
> > > !CONFIG_AMD_MEM_ENCRYPT, and preferably !sev_active() since the big
> > > array is used only if SEV is active.  This patch unconditionally
> > > defines hv_clock_dec but only frees it if CONFIG_AMD_MEM_ENCRYPT=y &&
> > > !mem_encrypt_active().
> >
> > Again we have to consider the bare metal scenario while doing this. The
> > aux array you proposed will be added in decrypted section only when
> > CONFIG_AMD_MEM_ENCRYPT=y.  If CONFIG_AMD_MEM_ENCRYPT=n then nothng
> > gets put in .data.decrypted section. At the runtime, if memory
> > encryption is active then .data.decrypted_hvclock will contains useful
> > data.
> >
> > The __decrypted attribute in "" when CONFIG_AMD_MEM_ENCRYPT=n.
>
> Right, but won't the data get dumped into the regular .bss in that
> case, i.e. needs to be freed?


Yes, the auxiliary array will dumped into the regular .bss when
CONFIG_AMD_MEM_ENCRYPT=n. Typically it will be few k, I am not
sure if its worth complicating the code to save those extra memory.
Most of the distro's have CONFIG_AMD_MEM_ENCRYPT=y anyways.