Re: [PATCH 6/7] x86/boot/compressed/64: Check SEV encryption in 32-bit boot-path

[Tom Lendacky]

On 2/10/21 10:47 AM, Dave Hansen wrote:
> On 2/10/21 2:21 AM, Joerg Roedel wrote:
> > +	/* Store to memory and keep it in the registers */
> > +	movl	%eax, rva(sev_check_data)(%ebp)
> > +	movl	%ebx, rva(sev_check_data+4)(%ebp)
> > +
> > +	/* Enable paging to see if encryption is active */
> > +	movl	%cr0, %edx	/* Backup %cr0 in %edx */
> > +	movl	$(X86_CR0_PG | X86_CR0_PE), %ecx /* Enable Paging and Protected mode */
> > +	movl	%ecx, %cr0
> > +
> > +	cmpl	%eax, rva(sev_check_data)(%ebp)
> > +	jne	3f
> > +	cmpl	%ebx, rva(sev_check_data+4)(%ebp)
> > +	jne	3f
>
> Also, I know that turning paging on is a *BIG* barrier.  But, I didn't
> think it has any effect on the caches.
>
> I would expect that the underlying physical address of 'sev_check_data'
> would change when paging gets enabled because paging sets the C bit.
> So, how does the write of 'sev_check_data' get out of the caches and
> into memory where it can be read back with the new physical address?
>
> I think there's some bit of the SEV architecture that I'm missing.

Non-paging memory accesses are always considered private (APM Volume 2, 
15.34.4) and thus are cached with the C bit set.

Thanks,
Tom

>