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SubjectRe: [RFC v1 25/26] x86/tdx: Make DMA pages shared
On Thu, Apr 01, 2021 at 02:01:15PM -0700, Dave Hansen wrote:
> > +int tdx_map_gpa(phys_addr_t gpa, int numpages, bool private)
> > +{
> > + int ret, i;
> > +
> > + ret = __tdx_map_gpa(gpa, numpages, private);
> > + if (ret || !private)
> > + return ret;
> > +
> > + for (i = 0; i < numpages; i++)
> > + tdx_accept_page(gpa + i*PAGE_SIZE);
> > +
> > + return 0;
> > +}
>
> Please do something like this:
>
> enum tdx_max_type {
> TDX_MAP_PRIVATE,
> TDX_MAP_SHARED
> }
>
> Then, your calls will look like:
>
> tdx_map_gpa(gpa, nr, TDX_MAP_SHARED);
>
> instead of:
>
> tdx_map_gpa(gpa, nr, false);

Okay, makes sense.

> > static __cpuidle void tdx_halt(void)
> > {
> > register long r10 asm("r10") = TDVMCALL_STANDARD;
> > diff --git a/arch/x86/mm/mem_encrypt_common.c b/arch/x86/mm/mem_encrypt_common.c
> > index 964e04152417..b6d93b0c5dcf 100644
> > --- a/arch/x86/mm/mem_encrypt_common.c
> > +++ b/arch/x86/mm/mem_encrypt_common.c
> > @@ -15,9 +15,9 @@
> > bool force_dma_unencrypted(struct device *dev)
> > {
> > /*
> > - * For SEV, all DMA must be to unencrypted/shared addresses.
> > + * For SEV and TDX, all DMA must be to unencrypted/shared addresses.
> > */
> > - if (sev_active())
> > + if (sev_active() || is_tdx_guest())
> > return true;
> >
> > /*
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index 16f878c26667..6f23a9816ef0 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -27,6 +27,7 @@
> > #include <asm/proto.h>
> > #include <asm/memtype.h>
> > #include <asm/set_memory.h>
> > +#include <asm/tdx.h>
> >
> > #include "../mm_internal.h"
> >
> > @@ -1977,8 +1978,8 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
> > struct cpa_data cpa;
> > int ret;
> >
> > - /* Nothing to do if memory encryption is not active */
> > - if (!mem_encrypt_active())
> > + /* Nothing to do if memory encryption and TDX are not active */
> > + if (!mem_encrypt_active() && !is_tdx_guest())
> > return 0;
>
> So, this is starting to look like the "enc" naming is wrong, or at least
> a little misleading. Should we be talking about "protection" or
> "guards" or something?

Are you talking about the function argument or function name too?

> > /* Should not be working on unaligned addresses */
> > @@ -1988,8 +1989,14 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
> > memset(&cpa, 0, sizeof(cpa));
> > cpa.vaddr = &addr;
> > cpa.numpages = numpages;
> > - cpa.mask_set = enc ? __pgprot(_PAGE_ENC) : __pgprot(0);
> > - cpa.mask_clr = enc ? __pgprot(0) : __pgprot(_PAGE_ENC);
> > + if (is_tdx_guest()) {
> > + cpa.mask_set = __pgprot(enc ? 0 : tdx_shared_mask());
> > + cpa.mask_clr = __pgprot(enc ? tdx_shared_mask() : 0);
> > + } else {
> > + cpa.mask_set = __pgprot(enc ? _PAGE_ENC : 0);
> > + cpa.mask_clr = __pgprot(enc ? 0 : _PAGE_ENC);
> > + }
>
> OK, this is too hideous to live. It sucks that the TDX and SEV/SME bits
> are opposite polarity, but oh well.
>
> To me, this gets a lot clearer, and opens up room for commenting if you
> do something like:
>
> if (is_tdx_guest()) {
> mem_enc_bits = 0;
> mem_plain_bits = tdx_shared_mask();
> } else {
> mem_enc_bits = _PAGE_ENC;
> mem_plain_bits = 0
> }
>
> if (enc) {
> cpa.mask_set = mem_enc_bits;
> cpa.mask_clr = mem_plain_bits; // clear "plain" bits
> } else {
>
> cpa.mask_set = mem_plain_bits;
> cpa.mask_clr = mem_enc_bits; // clear encryption bits
> }

I'm not convinced that your approach it clearer. If you add the missing
__pgprot() it going to as ugly as the original.

But if a maintainer wants... :)

> > cpa.pgd = init_mm.pgd;
> >
> > /* Must avoid aliasing mappings in the highmem code */
> > @@ -1999,7 +2006,8 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
> > /*
> > * Before changing the encryption attribute, we need to flush caches.
> > */
> > - cpa_flush(&cpa, !this_cpu_has(X86_FEATURE_SME_COHERENT));
> > + if (!enc || !is_tdx_guest())
> > + cpa_flush(&cpa, !this_cpu_has(X86_FEATURE_SME_COHERENT));
>
> That "!enc" looks wrong to me. Caches would need to be flushed whenever
> encryption attributes *change*, not just when they are set.
>
> Also, cpa_flush() flushes caches *AND* the TLB. How does TDX manage to
> not need TLB flushes?

I will double-check everthing, but I think we can skip *both* cpa_flush()
for private->shared conversion. VMM and TDX module will take care about
TLB and cache flush in response to MapGPA TDVMCALL.

> > ret = __change_page_attr_set_clr(&cpa, 1);
> >
> > @@ -2012,6 +2020,11 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
> > */
> > cpa_flush(&cpa, 0);
> >
> > + if (!ret && is_tdx_guest()) {
> > + ret = tdx_map_gpa(__pa(addr), numpages, enc);
> > + // XXX: need to undo on error?
> > + }
>
> Time to fix this stuff up if you want folks to take this series more
> seriously.

My bad, will fix it.

--
Kirill A. Shutemov

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