Re: [PATCH] x86/kvm: Override default caching mode for SEV-SNP and TDX

[Kirill A. Shutemov]

On Tue, Oct 15, 2024 at 12:12:51PM +0200, Jürgen Groß wrote:
> On 15.10.24 11:58, Kirill A. Shutemov wrote:
> > AMD SEV-SNP and Intel TDX have limited access to MTRR: either it is not
> > advertised in CPUID or it cannot be programmed (on TDX, due to #VE on
> > CR0.CD clear).
> > 
> > This results in guests using uncached mappings where it shouldn't and
> > pmd/pud_set_huge() failures due to non-uniform memory type reported by
> > mtrr_type_lookup().
> > 
> > Override MTRR state, making it WB by default as the kernel does for
> > Hyper-V guests.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Suggested-by: Binbin Wu <binbin.wu@xxxxxxxxx>
> > Cc: Juergen Gross <jgross@xxxxxxxx>
> > Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > ---
> >   arch/x86/kernel/kvm.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 263f8aed4e2c..21e9e4845354 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -37,6 +37,7 @@
> >   #include <asm/apic.h>
> >   #include <asm/apicdef.h>
> >   #include <asm/hypervisor.h>
> > +#include <asm/mtrr.h>
> >   #include <asm/tlb.h>
> >   #include <asm/cpuidle_haltpoll.h>
> >   #include <asm/ptrace.h>
> > @@ -980,6 +981,9 @@ static void __init kvm_init_platform(void)
> >   	}
> >   	kvmclock_init();
> >   	x86_platform.apic_post_init = kvm_apic_init;
> > +
> > +	/* Set WB as the default cache mode for SEV-SNP and TDX */
> > +	mtrr_overwrite_state(NULL, 0, MTRR_TYPE_WRBACK);
> 
> Do you really want to do this for _all_ KVM guests?
> 
> I'd expect this call to be conditional on TDX or SEV-SNP.

mtrr_overwrite_state() checks it internally.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov