linux-kernel - Re: [PATCH v6 05/10] x86/tdx: Handle port I/O

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Date:   Thu, 23 Sep 2021 12:59:43 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 05/10] x86/tdx: Handle port I/O

On 9/23/21 12:24 PM, Kuppuswamy, Sathyanarayanan wrote:
> 
> 
> On 9/23/21 9:32 AM, Tom Lendacky wrote:
>> On 9/22/21 5:52 PM, Kuppuswamy Sathyanarayanan wrote:
>>> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>>>
>>> TDX hypervisors cannot emulate instructions directly. This includes
>>> port IO which is normally emulated in the hypervisor. All port IO
>>> instructions inside TDX trigger the #VE exception in the guest and
>>> would be normally emulated there.
>>>
>>> Also string I/O is not supported in TDX guest. So, unroll the string
>>> I/O operation into a loop operating on one element at a time. This
>>> method is similar to AMD SEV, so just extend the support for TDX guest
>>> platform.
>>>
>>> Add a new confidential guest flag CC_ATTR_GUEST_UNROLL_STRING_IO to
>>> add string unroll support in asm/io.h
>>>
>>> Co-developed-by: Kuppuswamy Sathyanarayanan 
>>> <sathyanarayanan.kuppuswamy@...ux.intel.com>
>>> Signed-off-by: Kuppuswamy Sathyanarayanan 
>>> <sathyanarayanan.kuppuswamy@...ux.intel.com>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>>> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
>>> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
>>> ---
>>>
>>> Changes since v5:
>>>   * Changed prot_guest_has() to cc_platform_has().
>>>
>>> Changes since v4:
>>>   * Changed order of variable declaration in tdx_handle_io().
>>>   * Changed tdg_* prefix with tdx_*.
>>>
>>> Changes since v3:
>>>   * Included PATTR_GUEST_UNROLL_STRING_IO protected guest flag
>>>     addition change in this patch.
>>>   * Rebased on top of Tom Lendacks protected guest change.
>>>
>>> Changes since v2:
>>>   * None
>>>
>>> Changes since v1:
>>>   * Fixed comments for tdg_handle_io().
>>>   * Used _tdx_hypercall() instead of __tdx_hypercall() in tdg_handle_io().
>>>
>>>   arch/x86/include/asm/io.h   |  7 +++++--
>>>   arch/x86/kernel/cpu/intel.c |  1 +
>>>   arch/x86/kernel/tdx.c       | 35 +++++++++++++++++++++++++++++++++++
>>>   include/linux/cc_platform.h | 11 +++++++++++
>>>   4 files changed, 52 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
>>> index fa6aa43e5dc3..67e0c4a0a0f4 100644
>>> --- a/arch/x86/include/asm/io.h
>>> +++ b/arch/x86/include/asm/io.h
>>> @@ -40,6 +40,7 @@
>>>   #include <linux/string.h>
>>>   #include <linux/compiler.h>
>>> +#include <linux/cc_platform.h>
>>>   #include <asm/page.h>
>>>   #include <asm/tdx.h>
>>>   #include <asm/early_ioremap.h>
>>> @@ -310,7 +311,8 @@ static inline unsigned type in##bwl##_p(int 
>>> port)            \
>>>                                       \
>>>   static inline void outs##bwl(int port, const void *addr, unsigned 
>>> long count) \
>>>   {                                    \
>>> -    if (sev_key_active()) {                        \ > +    if 
>>> (sev_key_active() ||                        \
>>> +        cc_platform_has(CC_ATTR_GUEST_UNROLL_STRING_IO)) {        \
>>
>> Would it make sense to make sev_key_active() and sev_enable_key generic 
>> and just re-use those instead of adding CC_ATTR_GUEST_UNROLL_STRING_IO 
>> and having multiple conditions here?
>>
>> You can set the key in the TDX init routine just like SEV does.
> 
> Any reason for using sev_enable_key over CC attribute? IMO, CC attribute 
> exist
> to generalize the common feature code. My impression is SEV is specific to 
> AMD
> code.

When the SEV series was initially submitted, it originally did an 
sev_active() check. For various reasons a static key and the 
sev_key_active() call was requested.

My suggestion was to change the name to something that doesn't have 
SEV/sev in it that can be used by both SEV and TDX. The sev_enable_key can 
be moved to a common file (maybe cc_platform.c) and renamed. Then 
arch/x86/include/asm/io.h can change the #ifdef from 
CONFIG_AMD_MEM_ENCRYPT to CONFIG_ARCH_HAS_CC_PLATFORM.

Not sure if anyone else feels the same, though, so just my suggestion.

Thanks,
Tom

> 
>>
>> Thanks,
>> Tom
>>
>