Re: [Xen-devel] [PATCH v4 4/7] x86/HVM: implement memory read caching for insn emulation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 31/01/2020 16:44, Jan Beulich wrote:
> Emulation requiring device model assistance uses a form of instruction
> re-execution, assuming that the second (and any further) pass takes
> exactly the same path. This is a valid assumption as far as use of CPU
> registers goes (as those can't change without any other instruction
> executing in between), but is wrong for memory accesses.

This statement isn't quite accurate.  Various hypercalls and IPIs can
play with GPR state in the middle of emulation.

What matters is that the GPR values aren't expected to change, and a
guest gets to keep all the pieces if they try to play games in this area.

>  In particular
> it has been observed that Windows might page out buffers underneath an
> instruction currently under emulation (hitting between two passes). If
> the first pass read a memory operand successfully, any subsequent pass
> needs to get to see the exact same value.
>
> Introduce a cache to make sure above described assumption holds. This
> is a very simplistic implementation for now: Only exact matches are
> satisfied (no overlaps or partial reads or anything); this is sufficient
> for the immediate purpose of making re-execution an exact replay. The
> cache also won't be used just yet for guest page walks; that'll be the
> subject of a subsequent change.
>
> With the cache being generally transparent to upper layers, but with it
> having limited capacity yet being required for correctness, certain
> users of hvm_copy_from_guest_*() need to disable caching temporarily,
> without invalidating the cache. Note that the adjustments here to
> hvm_hypercall() and hvm_task_switch() are benign at this point; they'll
> become relevant once we start to be able to emulate respective insns
> through the main emulator (and more changes will then likely be needed
> to nested code).
>
> As to the actual data page in this scenario, there are a couple of
> aspects to take into consideration:
> - We must be talking about an insn accessing two locations (two memory
>   ones, one of which is MMIO, or a memory and an I/O one).

Really?

We're talking about all instructions, even without any memory operands,
because at the very minimum we cache the instruction stream, and (with
the following patch), the pagewalk to it.

> - If the non I/O / MMIO side is being read, the re-read (if it occurs at
>   all) is having its result discarded, by taking the shortcut through
>   the first switch()'s STATE_IORESP_READY case in hvmemul_do_io(). Note
>   how, among all the re-issue sanity checks there, we avoid comparing
>   the actual data.
> - If the non I/O / MMIO side is being written, it is the OSes
>   responsibility to avoid actually moving page contents to disk while
>   there might still be a write access in flight - this is no different
>   in behavior from bare hardware.
> - Read-modify-write accesses are, as always, complicated, and while we
>   deal with them better nowadays than we did in the past, we're still
>   not quite there to guarantee hardware like behavior in all cases
>   anyway. Nothing is getting worse by the changes made here, afaict.
>
> In __hvm_copy() also reduce p's scope and change its type to void *.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> TBD: In principle the caching here yields unnecessary the one used for
>      insn bytes (vio->mmio_insn{,_bytes}. However, to seed the cache
>      with the data SVM may have made available, we'd have to also know
>      the corresponding GPA. It's not safe, however, to re-walk the page
>      tables to find out, as the page tables may have changed in the
>      meantime. Therefore I guess we need to keep the duplicate
>      functionality for now. A possible solution to this could be to use
>      a physical-address-based cache for page table accesses (and looking
>      forward also e.g. SVM/VMX insn emulation), and a linear-address-
>      based one for all other reads.

Splitting caching like that will re-introduce the same bugs I pointed
out in earlier revisions of this series.  It is not correct to have
multiple (and therefore, non-coherent) caches of memory.

The AMD instruction stream bytes support is by no means perfect either. 
It doesn't function properly when SMAP is active (Erratum #1096), which
is actually caused by the instruction stream being re-fetched at vmexit
time, with a data side access (hence the interaction with SMAP).

Just as with the emulation for non-Nrips hardware, the contents of the
instruction buffer (if valid) need cross referencing with the vmexit
reason to spot races, and the same checking would make it safe to
deduplicate the caching.

> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -28,6 +28,17 @@
>  #include <asm/iocap.h>
>  #include <asm/vm_event.h>
>  
> +struct hvmemul_cache
> +{

This needs a comment (see below.)

> +    unsigned int num_ents;
> +    unsigned int max_ents;
> +    struct {
> +        paddr_t gpa:PADDR_BITS;
> +        unsigned int size:BITS_PER_LONG - PADDR_BITS;

This would probably result in rather better code if size was 8 bits
rather than 12.

Longer term, the size field should disappear and functionality adjusted
to always read aligned 8 (or larger) byte values.  That way, the cache
behaves much more like caches in real processors.

> @@ -2838,6 +2868,123 @@ void hvm_dump_emulation_state(const char
>             hvmemul_ctxt->insn_buf);
>  }
>  
> +int hvmemul_cache_init(struct vcpu *v)
> +{
> +    /*
> +     * No insn can access more than 16 independent linear addresses (AVX512F
> +     * scatters/gathers being the worst).

It is at least 5 more than the worse case number of regular operands, to
cover exceptions, and therefore accesses into the IDT/GDT/LDT and TR and
stack.

You can manage that with a 32bit guest with an exception, %cs and %ss in
different tables, and having to cross a page boundary when pushing the
esp0 exception frame.

It is one fewer generally in 64bit mode, because there are no %ss
references to deal with, but both cases get even more complicated for a
contributory exception, as the second %cs can be different to %cs for
the first exception.

>  Each such linear range can span a
> +     * page boundary, i.e. may require two page walks. Account for each insn
> +     * byte individually.

Why are we counting 5 entries for every instruction byte?  Given an
8-byte maximum cached size, we need at most 2 entries to cover a single
instruction.

> +     */
> +    const unsigned int nents = (CONFIG_PAGING_LEVELS + 1) *
> +                               (MAX_INST_LEN + 16 * 2);
> +    struct hvmemul_cache *cache = xmalloc_flex_struct(struct hvmemul_cache,
> +                                                      ents, nents);
> +
> +    if ( !cache )
> +        return -ENOMEM;
> +
> +    /* Cache is disabled initially. */
> +    cache->num_ents = nents + 1;
> +    cache->max_ents = nents;

(Below:) You need to document the internal semantics (probably best in
the struct definition.)

I thought I'd reverse engineered how disabling works (starting from the
completely obscure hvmemul_cache_disabled()), but given that the cache
is apparently disabled by default, I now can't figure out how it ever
gets enabled.

I can't see any code path where num_ents ends up lower than max_ents,
and the read/write helpers below don't take their early exit path.

> +bool hvmemul_read_cache(const struct vcpu *v, paddr_t gpa,
> +                        void *buffer, unsigned int size)
> +{
> +    const struct hvmemul_cache *cache = v->arch.hvm.hvm_io.cache;
> +    unsigned int i;
> +
> +    /* Cache unavailable? */
> +    if ( cache->num_ents > cache->max_ents )
> +        return false;
> +
> +    while ( size > sizeof(cache->ents->data) )
> +    {
> +        i = gpa & (sizeof(cache->ents->data) - 1)
> +            ? -gpa & (sizeof(cache->ents->data) - 1)
> +            : sizeof(cache->ents->data);
> +        if ( !hvmemul_read_cache(v, gpa, buffer, i) )
> +            return false;

What is this call trying to achieve?

> +        gpa += i;
> +        buffer += i;
> +        size -= i;
> +    }
> +
> +    for ( i = 0; i < cache->num_ents; ++i )
> +        if ( cache->ents[i].gpa == gpa && cache->ents[i].size == size )

With num_ents currently topping out at 235 (and needing to increase
anyway), this can end up being a very long loop.

Given that it will be populated with higher level PTEs to being with,
the common case of accessing real data will be at the end of the O(n)
search rather than the start.

The cache is a single block of memory, so it is almost certainly better
to keep it sorted.  That said, it is problematic due to the overlap of
sizes, which I'm still concerned concerned about generally.

> --- a/xen/include/asm-x86/hvm/emulate.h
> +++ b/xen/include/asm-x86/hvm/emulate.h
> @@ -13,6 +13,7 @@
>  #define __ASM_X86_HVM_EMULATE_H__
>  
>  #include <xen/err.h>
> +#include <xen/sched.h>
>  #include <asm/hvm/hvm.h>
>  #include <asm/x86_emulate.h>
>  
> @@ -96,6 +97,31 @@ int hvmemul_do_pio_buffer(uint16_t port,
>                            uint8_t dir,
>                            void *buffer);
>  
> +#ifdef CONFIG_HVM

This needs a comment block stating, at a minimum, the semantics and
expected use for the cache.

Otherwise, this is ~150 lines of totally undocumented and practically
uncommented, critical and subtle infrastructure.

~Andrew

> +int __must_check hvmemul_cache_init(struct vcpu *v);
> +static inline void hvmemul_cache_destroy(struct vcpu *v)
> +{
> +    XFREE(v->arch.hvm.hvm_io.cache);
> +}
> +bool hvmemul_read_cache(const struct vcpu *, paddr_t gpa,
> +                        void *buffer, unsigned int size);
> +void hvmemul_write_cache(const struct vcpu *, paddr_t gpa,
> +                         const void *buffer, unsigned int size);
> +unsigned int hvmemul_cache_disable(struct vcpu *);
> +void hvmemul_cache_restore(struct vcpu *, unsigned int token);
> +/* For use in ASSERT()s only: */
> +static inline bool hvmemul_cache_disabled(struct vcpu *v)
> +{
> +    return hvmemul_cache_disable(v) == hvmemul_cache_disable(v);
> +}
> +#else
> +static inline bool hvmemul_read_cache(const struct vcpu *v, paddr_t gpa,
> +                                      void *buf,
> +                                      unsigned int size) { return false; }
> +static inline void hvmemul_write_cache(const struct vcpu *v, paddr_t gpa,
> +                                       const void *buf, unsigned int size) {}
> +#endif
> +
>  void hvm_dump_emulation_state(const char *loglvl, const char *prefix,
>                                struct hvm_emulate_ctxt *hvmemul_ctxt, int rc);
>  

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