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Date:   Fri, 20 Jan 2023 21:15:10 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Michael Kelley <mikelley@microsoft.com>
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Subject: Re: [PATCH v5 06/14] x86/ioremap: Support hypervisor specified range
 to map as encrypted
Message-ID: <Y8r2TjW/R3jymmqT@zn.tnic>
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On Thu, Jan 12, 2023 at 01:42:25PM -0800, Michael Kelley wrote:
> In a AMD SEV-SNP VM using vTOM, devices in MMIO space may be provided by
> the paravisor and need to be mapped as encrypted.  Provide a function
> for the hypervisor to specify the address range for such devices.
> In __ioremap_caller(), map addresses in this range as encrypted.
> 
> Only a single range is supported. If multiple devices need to be
> mapped encrypted, the paravisor must place them within the single
> contiguous range.

This already is starting to sound insufficient and hacky. And it also makes
CC_ATTR_ACCESS_IOAPIC_ENCRYPTED insufficient either.

So, the situation we have is, we're a SEV-SNP VM using vTOM. Which means,
MSR_AMD64_SEV[3] = 1. Or SEV_FEATURES[1], alternatively - same thing.

That MSR cannot be intercepted by the HV and we use it extensively in Linux when
it runs as a SEV-* guest. And I had asked this before, during review, but why
aren't you checking this bit above when you wanna do vTOM-specific work?

Because then you can do that check and

1. map the IO-APIC encrypted
2. map MMIO space of devices from the driver encrypted too
3. ...

and so on.

And you won't need those other, not as nice things...

Hmmm.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette