linux-kernel - Re: [PATCH 7/9] x86/sev-es: Include XSS value in GHCB CPUID request

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Message-ID: <015ee02a072460b9e844278f5cc4c3b3618f5e9e.camel@redhat.com>
Date:   Thu, 02 Nov 2023 20:14:05 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Borislav Petkov <bp@...en8.de>, John Allen <john.allen@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, weijiang.yang@...el.com,
        rick.p.edgecombe@...el.com, seanjc@...gle.com, x86@...nel.org,
        thomas.lendacky@....com
Subject: Re: [PATCH 7/9] x86/sev-es: Include XSS value in GHCB CPUID request

On Tue, 2023-10-17 at 20:49 +0200, Borislav Petkov wrote:
> On Tue, Oct 17, 2023 at 01:12:30PM -0500, John Allen wrote:
> > I looked into using __rdmsr in an earlier revision of the patch, but
> > found that it causes a build warning:
> > 
> > ld: warning: orphan section `__ex_table' from `arch/x86/boot/compressed/sev.o' being placed in section `__ex_table'
> > 
> > This is due to the __ex_table section not being used during
> > decompression boot. Do you know of a way around this?
> 
> Yeah, arch/x86/boot/msr.h.
> 
> We did those a while ago. I guess they could be moved to
> asm/shared/msr.h and renamed to something that is not a
> "boot_" prefix.
> 
> Something like
> 
> rdmsr_without_any_exception_handling_and_other_gunk_don_t_you_even_think_of_adding()
> 
> ...
> 
> But srsly:
> 
> raw_rdmsr()
> raw_wrmsr()
> 
> should be good, as tglx suggests offlist.
> 
> You can do that in one patch and prepend your set with it.
> 
> Thx.
> 


Assuming that we will actually allow the guest to read the IA32_XSS, then it is correct,
but otherwise we will need to return some cached value of IA32_XSS,
the same as the guest wrote last time.

Or another option: KVM can cache the last value that the guest wrote to IA32_XSS (I assume that
the guest can write msrs by sharing the address and value via GHCB), and then use it
when it constructs the CPUID.

Best regards,
	Maxim Levitsky