Re: [PATCH v2] KVM: SVM: Override default MMIO mask if memory encryption is enabled

Tom Lendacky
2020-01-07T22:55:21Z

On 1/7/20 4:28 PM, Sean Christopherson wrote:
> On Tue, Jan 07, 2020 at 02:16:37PM -0600, Tom Lendacky wrote:
>> On 1/6/20 5:38 PM, Sean Christopherson wrote:
>>> On Mon, Jan 06, 2020 at 05:14:04PM -0600, Tom Lendacky wrote:
>>>> On 1/6/20 4:49 PM, Sean Christopherson wrote:
>>>>> This doesn't handle the case where x86_phys_bits _isn't_ reduced by SME/SEV
>>>>> on a future processor, i.e. x86_phys_bits==52.
>>>>
>>>> Not sure I follow. If MSR_K8_SYSCFG_MEM_ENCRYPT is set then there will
>>>> always be a reduction in physical addressing (so I'm told).
>>>
>>> Hmm, I'm going off APM Vol 2, which states, or at least strongly implies,
>>> that reducing the PA space is optional. Section 7.10.2 is especially
>>> clear on this:
>>>
>>> In implementations where the physical address size of the processor is
>>> reduced when memory encryption features are enabled, software must
>>> ensure it is executing from addresses where these upper physical address
>>> bits are 0 prior to setting SYSCFG[MemEncryptionModEn].
>>
>> It's probably not likely, but given what is stated, I can modify my patch
>> to check for a x86_phys_bits == 52 and skip the call to set the mask, eg:
>>
>> if (msr & MSR_K8_SYSCFG_MEM_ENCRYPT &&
>> boot_cpu_data.x86_phys_bits < 52) {
>>
>>>
>>> But, hopefully the other approach I have in mind actually works, as it's
>>> significantly less special-case code and would naturally handle either
>>> case, i.e. make this a moot point.
>>
>> I'll hold off on the above and wait for your patch.
>
> Sorry for the delay, this is a bigger mess than originally thought. Or
> I'm completely misunderstanding the issue, which is also a distinct
> possibility :-)
>
> Due to KVM activating its L1TF mitigation irrespective of whether the CPU
> is whitelisted as not being vulnerable to L1TF, simply using 86_phys_bits
> to avoid colliding with the C-bit isn't sufficient as the L1TF mitigation
> uses those first five reserved PA bits to store the MMIO GFN. Setting
> BIT(x86_phys_bits) for all MMIO sptes would cause it to be interpreted as
> a GFN bit when the L1TF mitigation is active and lead to bogus MMIO.

The L1TF mitigation only gets applied when:
boot_cpu_data.x86_cache_bits < 52 - shadow_nonpresent_or_rsvd_mask_len

and with shadow_nonpresent_or_rsvd_mask_len = 5, that means that means
boot_cpu_data.x86_cache_bits < 47.

On AMD processors that support memory encryption, the x86_cache_bits value
is not adjusted, just the x86_phys_bits. So for AMD processors that have
memory encryption support, this value will be at least 48 and therefore
not activate the L1TF mitigation.

>
> The only sane approach I can think of is to activate the L1TF mitigation
> based on whether the CPU is vulnerable to L1TF, as opposed to activating> the mitigation purely based on the max PA of the CPU. Since all CPUs that
> support SME/SEV are whitelisted as NO_L1TF, the L1TF mitigation and C-bit
> should never be active at the same time.

There is still the issue of setting a single bit that can conflict with
the C-bit. As it is today, if the C-bit were to be defined as bit 51, then
KVM would not take a nested page fault and MMIO would be broken.

Thanks,
Tom

>
> Patch should be incoming soon...
>