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

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Date:   Wed, 10 Feb 2021 14:44:15 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org
Cc:     Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 6/7] x86/boot/compressed/64: Check SEV encryption in
 32-bit boot-path

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

>