Re: [RFC v2-fix-v1 3/3] x86/tdx: Handle port I/O

From: Dan Williams
Date: Sat Jun 05 2021 - 15:01:39 EST

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On Wed, May 26, 2021 at 9:24 PM Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> wrote:
>
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> 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.
>
> For the really early code in the decompressor, #VE cannot be
> used because the IDT needed for handling the exception is not
> set-up, and some other infrastructure needed by the handler
> is missing. So to support port IO in decompressor code, add
> support for paravirt based I/O port virtualization.
>
> 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.

Given early port IO is broken out in its own previous I think it makes
sense to break out the decompressor port IO enabling from final
runtime port IO support.

The argument in the previous patch about using #VE emulation in the
early code was collisions with trace and printk support in the "fully
featured" #VE handler later in the series. My interpretation of that
collision was due to the possibility of the #VE handler going into
infinite recursion if a printk in the handler triggered port IO. It
seems I do not have the right picture of the constraints. Given the
runtime kernel can direct replace in/out macros I would expect a
statement of the tradeoff with #VE emulation and why the post
decompressor code is still using emulation.

>
> Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
> arch/x86/boot/compressed/Makefile | 1 +
> arch/x86/boot/compressed/tdcall.S | 3 ++
> arch/x86/boot/compressed/tdx.c | 28 ++++++++++++++++++
> arch/x86/include/asm/io.h | 7 +++--
> arch/x86/include/asm/tdx.h | 47 ++++++++++++++++++++++++++++++-
> arch/x86/kernel/tdx.c | 39 +++++++++++++++++++++++++
> 6 files changed, 122 insertions(+), 3 deletions(-)
> create mode 100644 arch/x86/boot/compressed/tdcall.S
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index a2554621cefe..a944a2038797 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -97,6 +97,7 @@ endif
>
> vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
> vmlinux-objs-$(CONFIG_INTEL_TDX_GUEST) += $(obj)/tdx.o
> +vmlinux-objs-$(CONFIG_INTEL_TDX_GUEST) += $(obj)/tdcall.o
>
> vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
> efi-obj-$(CONFIG_EFI_STUB) = $(objtree)/drivers/firmware/efi/libstub/lib.a
> diff --git a/arch/x86/boot/compressed/tdcall.S b/arch/x86/boot/compressed/tdcall.S
> new file mode 100644
> index 000000000000..aafadc136c88
> --- /dev/null
> +++ b/arch/x86/boot/compressed/tdcall.S
> @@ -0,0 +1,3 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#include "../../kernel/tdcall.S"
> diff --git a/arch/x86/boot/compressed/tdx.c b/arch/x86/boot/compressed/tdx.c
> index 0a87c1775b67..cb20962c7da6 100644
> --- a/arch/x86/boot/compressed/tdx.c
> +++ b/arch/x86/boot/compressed/tdx.c
> @@ -4,6 +4,8 @@
> */
>
> #include <asm/tdx.h>
> +#include <asm/vmx.h>
> +#include <vdso/limits.h>
>
> static int __ro_after_init tdx_guest = -1;
>
> @@ -30,3 +32,29 @@ bool is_tdx_guest(void)
> return !!tdx_guest;
> }
>
> +/*
> + * Helper function used for making hypercall for "out"
> + * instruction. It will be called from __out IO
> + * macro (in tdx.h).
> + */
> +void tdg_out(int size, int port, unsigned int value)
> +{
> + __tdx_hypercall(EXIT_REASON_IO_INSTRUCTION, size, 1,
> + port, value, NULL);
> +}
> +
> +/*
> + * Helper function used for making hypercall for "in"
> + * instruction. It will be called from __in IO macro
> + * (in tdx.h). If IO is failed, it will return all 1s.
> + */
> +unsigned int tdg_in(int size, int port)
> +{
> + struct tdx_hypercall_output out = {0};
> + int err;
> +
> + err = __tdx_hypercall(EXIT_REASON_IO_INSTRUCTION, size, 0,
> + port, 0, &out);
> +
> + return err ? UINT_MAX : out.r11;
> +}

The previous patch open coded tdg_{in,out} and this one provides
helpers. I think at a minimum they should be consistent and pick one
style.

> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
> index ef7a686a55a9..daa75c8eef5d 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/protected_guest.h>
> #include <asm/page.h>
> #include <asm/early_ioremap.h>
> #include <asm/pgtable_types.h>
> @@ -309,7 +310,8 @@ 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() || \
> + protected_guest_has(VM_UNROLL_STRING_IO)) { \
> unsigned type *value = (unsigned type *)addr; \
> while (count) { \
> out##bwl(*value, port); \
> @@ -325,7 +327,8 @@ static inline void outs##bwl(int port, const void *addr, unsigned long count) \
> \
> static inline void ins##bwl(int port, void *addr, unsigned long count) \
> { \
> - if (sev_key_active()) { \
> + if (sev_key_active() || \
> + protected_guest_has(VM_UNROLL_STRING_IO)) { \
> unsigned type *value = (unsigned type *)addr; \
> while (count) { \
> *value = in##bwl(port); \
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index e880a9dd40d3..6ba2dcea533f 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -5,6 +5,8 @@
>
> #define TDX_CPUID_LEAF_ID 0x21
>
> +#ifndef __ASSEMBLY__
> +
> #ifdef CONFIG_INTEL_TDX_GUEST
>
> #include <asm/cpufeature.h>
> @@ -74,6 +76,48 @@ u64 __tdx_hypercall(u64 fn, u64 r12, u64 r13, u64 r14, u64 r15,
> bool tdx_protected_guest_has(unsigned long flag);
> bool tdg_early_handle_ve(struct pt_regs *regs);
>
> +void tdg_out(int size, int port, unsigned int value);
> +unsigned int tdg_in(int size, int port);
> +
> +/* Helper function for converting {b,w,l} to byte size */
> +static inline int tdx_get_iosize(char *str)
> +{
> + if (str[0] == 'w')
> + return 2;
> + else if (str[0] == 'l')
> + return 4;
> +
> + return 1;
> +}

This seems like an unnecessary novelty. The BUILDIO() macro in
arch/x86/include/asm/io.h takes a type argument, why can't the size be
explicitly specified rather than inferred from string parsing?

> +
> +/*
> + * To support I/O port access in decompressor or early kernel init
> + * code, since #VE exception handler cannot be used, use paravirt
> + * model to implement __in/__out macros which will in turn be used
> + * by in{b,w,l}()/out{b,w,l} I/O helper macros used in kernel. You
> + * can find the __in/__out macro usage in arch/x86/include/asm/io.h
> + */
> +#ifdef BOOT_COMPRESSED_MISC_H
> +#define __out(bwl, bw) \
> +do { \
> + if (is_tdx_guest()) { \
> + tdg_out(tdx_get_iosize(#bwl), port, value); \
> + } else { \
> + asm volatile("out" #bwl " %" #bw "0, %w1" : : \
> + "a"(value), "Nd"(port)); \
> + } \
> +} while (0)
> +#define __in(bwl, bw) \
> +do { \
> + if (is_tdx_guest()) { \
> + value = tdg_in(tdx_get_iosize(#bwl), port); \
> + } else { \
> + asm volatile("in" #bwl " %w1, %" #bw "0" : \
> + "=a"(value) : "Nd"(port)); \
> + } \
> +} while (0)
> +#endif
> +
> #else // !CONFIG_INTEL_TDX_GUEST
>
> static inline bool is_tdx_guest(void)
> @@ -161,6 +205,7 @@ static inline long tdx_kvm_hypercall4(unsigned int nr, unsigned long p1,
> {
> return -ENODEV;
> }
> -#endif /* CONFIG_INTEL_TDX_GUEST_KVM */
>
> +#endif /* CONFIG_INTEL_TDX_GUEST_KVM */
> +#endif /* __ASSEMBLY__ */
> #endif /* _ASM_X86_TDX_H */
> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> index ca3442b7accf..4a84487ee8ff 100644
> --- a/arch/x86/kernel/tdx.c
> +++ b/arch/x86/kernel/tdx.c
> @@ -202,6 +202,42 @@ static void tdg_handle_cpuid(struct pt_regs *regs)
> regs->dx = out.r15;
> }
>
> +void tdg_out(int size, int port, unsigned int value)
> +{
> + tdx_hypercall(EXIT_REASON_IO_INSTRUCTION, size, 1, port, value);
> +}
> +
> +unsigned int tdg_in(int size, int port)
> +{
> + struct tdx_hypercall_output out = {0};
> + u64 err;
> +
> + err = __tdx_hypercall(EXIT_REASON_IO_INSTRUCTION, size, 0,
> + port, 0, &out);
> +
> + return err ? UINT_MAX : out.r11;
> +}
> +
> +static void tdg_handle_io(struct pt_regs *regs, u32 exit_qual)
> +{
> + bool string = exit_qual & 16;
> + int out, size, port;
> +
> + /* I/O strings ops are unrolled at build time. */
> + BUG_ON(string);

...and here is why I think the WBINVD patch is bogus, or at least
inconsistent with the decision taken here. If it's ok to BUG_ON
instructions that "can't happen" due to care taken to ensure build
time guarantees then it is ok to skip WBINVD handling with the same
care taken to prevent its usage at build time.

> +
> + out = VE_GET_IO_TYPE(exit_qual);
> + size = VE_GET_IO_SIZE(exit_qual);
> + port = VE_GET_PORT_NUM(exit_qual);
> +
> + if (out) {
> + tdg_out(size, port, regs->ax);
> + } else {
> + regs->ax &= ~GENMASK(8 * size, 0);
> + regs->ax |= tdg_in(size, port) & GENMASK(8 * size, 0);
> + }
> +}
> +
> unsigned long tdg_get_ve_info(struct ve_info *ve)
> {
> u64 ret;
> @@ -248,6 +284,9 @@ int tdg_handle_virtualization_exception(struct pt_regs *regs,
> case EXIT_REASON_CPUID:
> tdg_handle_cpuid(regs);
> break;
> + case EXIT_REASON_IO_INSTRUCTION:
> + tdg_handle_io(regs, ve->exit_qual);
> + break;
> default:
> pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
> return -EFAULT;
> --
> 2.25.1
>

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